I would argue that those bi-fold doors don't actually save space. you need to have the space around those doors free of clutter, otherwise you can't open the door. I've gotten rid of my bi-fold doors in favour of curtains.those are truly space saving.

I'm glad you asked because I've got the same problem, looking forward to the comments.

If you want to save space maybe sliding doors? Not my personal favorite but we just installed them in our master closet.

Or IKEA track doors/curtains. They can look pretty solid if done right.

It looks like you can find them at Lowes or order them thru Menards.

I did a search for bifold doors without louver and got quite a few hits. Some were like a french door with glass panes and actually kind of pretty.

Personally, I'd opt more toward a slider. Bifolds seem to come off track frequently. A slider with narrow panels would be my preference.

Just to clarify: by standard closet door, I mean 30" wide.
-Kay (original poster)

Here are some ideas for you:

This place has non-louver bifold doors. I'm not sure their sizes though.
http://www.doitbest.com/Bi-fold+and+closet+doors-Masonite-model--doitbest-sku-162280.dib

This site has a larger selection and the website is better organized.
http://www.midwestmanufacturing.com/MidwestManufacturing/productLine.do?groupId=0&productTypeId=1&lineId=14&Doors=Bi-Fold-Doors

Also, Here is an idea for taking the less expensive "plain" type doors and making them more modern. http://www.do-it-yourselfdesign.blogspot.com/2012/06/bi-fold-closet-door-makeover.html

I agree with the others, I have bifolds on every door, and I'm not really a fan. Mine look "modern" because they're completely smooth, hollow doors. Plain ol' builder grade. No moulding or anything. With the right finish and knobs, they could take on whatever look you want:
http://www.homedepot.com/Doors-Windows-Doors-Interior-Doors-Bi-fold-Doors/h_d1/N-5yc1vZapvv/R-100030484/h_d2/ProductDisplay?catalogId=10053#.UOs8tonjlYQ

as others have noted, you can get flush, hollow-core 1-3/8 bifold doors at most building supply yards (home depot, lowes, etc)... they'll need to be special ordered through the door/millwork folks. in most cases, they'll need to know the finished opening size and the factory will build to spec. they'll supply a top track and bottom mounting brackets. we used them in our laundry room and you can see them here, too...

Using this link from Menards, you can design your own... including glass and can choose a more simple profile. (http://www.menards.com/main/mastercraftDoorDesigner.html)

i just noticed that you're looking to replace a swing-door with bi-folds. they can indeed save space insofar as the swing (15in to be exact), but when folded, will limit access about 5 inches (where the door collapses). when you replace them, you'll also need/want to fill in the holes left by the strikeplate and hinges. on the hinges, some thin scraps of wood leveled off with some bondo should work nicely. for the hole/strikeplate, a plug of wood with bondo as filler should also work. best of luck.

I would caution you about getting this type of folding doors. My friend renovated his home and used them on two closet doors and they both ended up getting loose and falling off the track. They seem to be good quality and were bought from big box stores, I think the problem is just the type of door. If you still want to get folding doors, make sure the track at top AND bottom runs the entire lengh of the door and that will ensure that the doors aren't too heavy for the hinge at top and come loose.

You can also cut out the shutters and do a little DIY:
http://unpretentiousstyle.blogspot.com/2009/04/inexpensive-closet-doors-makeover.html
or here: http://www.hgtv.com/home-improvement/transparent-panels-for-bi-fold-doors/index.html

Make it look more modern and maybe even paint it a gloss white or gray.

@jasmine: worth noting... this type of door generally does not have a bottom track (the hollow-core type from the big-box). if properly installed and sized there should not be a problem with them falling out of alignment. we had some fairly large ones in our old place that performed quite well for the 5 years we were there. the pulls being located at the knuckle of the door also help with even distribution of force when opening. if you put a pull where a knob might typically go, the force needed to open can get torquey.

Oh how I hate bi-fold doors with their old look and rattling tracks, the way they block the ends space of the closet... I went with clever hinges from Sugatsune instead - I paid a premium but they sure are better than bi-folds.

https://www.youtube.com/watch?v=VTPTq7LNG1k

Back in 1996, I put bi-fold doors on my closet, and there they stayed, without troubles or issues until 2008, I wasn't crazy about the louvered doors, but I found the doors to be quite functional. I had a total gut-rehab on the house in 2008 and went with french-styled doors for the remodel of the bedroom and closet. They work fine, too.

Here is a photo of the bi-folds, taken about 3 years before we changed the doors:
http://www.flickr.com/photos/k2yhe/4556000052/in/set-72157623938569198

I replaced curtains with bi-fold doors and it looks sooo much cleaner for this particular space. Someone was giving them away online. They're louvered, which I like because I like the texture against the wall. They were too wide for the weird door size that I had, so we shaved some off the sides. We also didn't use the rails but instead used magnetized clasps in various places to keep them shut, so they can swing freely open. This works better for accessing the closet. I know this sounds like a bit of a hack - because it was! But it worked really well.

When you find the right bifold doors - don't put the hinges in the middle, put the hinges on the outer edges, so you end up with two 15" wide doors that open in the middle. When open, they will take up the same amount of room as a bifold, but you don't have to deal with the annoying bifold door track.

I hate my bifold doors, even though they are solid wood. They are all over my house. A huge pain....I would never put them in given the choice, they were here when we bought.

Weatherman those Sugatsune doors are cool, I've never seen those.

P.S. I'm actually thinking of trying a panel drapery system for my master bedroom closet, that's how much I dislike the bifolds.

I have bi-fold doors on the door to my bedroom closet. I inherited them when I bought by condo, but I actually rather like them as they're mirrored so when closed they reflect light around the room (the one window is directly opposite), and they're rather handy for making sure I don't look completely un-stylish before I go out the door in the mornings.
I'm sure they're available at any big-box building store (Home Depot, etc)

@ebm

The doors in your house are beautiful!

Can't see how bifold doors will save space, but if you go with the mirror suggestion they will make your space look bigger.

You've got the basic look I'm going for, redneckmodern. I was hoping for some leads on ones with frosted glass insets -- or something beyond what Blue and Orange carry.

In my experience, there are bifolds and bifolds. Some are ugly and falling off tracks; some work fine. I think it is the quality and sturdiness of what you purchase, as well as proper sizing and installation, that makes the difference. There is no reason that bifolds have to perform badly, just because they often do when poorly and cheaply installed.

Interior bifold doors do tend to come in various quality standards,you get what you pay for.Exterior bifold doors are also made from materials that can be considered, not fit for purpose.Strong durable and well tested units, will give you a trouble free life. The larger the opening the higher the quality you require, go fo the best you can afford and do go to a showroom to test them out. In the UK there are several showrooms around the country see which is nearest to you here