I love vintage maps, movie posters, and even print ads! I have an absinthe poster giclee on canvas hanging next to my liquor cabinet.

^ When my boyfriend moved into my apartment with me, we merged our enormous collections of vintage/foreign horror movie posters. My rule is "it doesn't get hung up unless it's in a frame," to keep the place from looking too dorm-y, but when all you have are giant theatrical one-sheets or odd-sized Polish/Korean/Mexican one-sheets (which are all different sizes), it gets really difficult to find stock frames! Paying out the nose to have them professionally framed is out of our price range - we have something like 10-12 still waiting on frames!

Is there anywhere that one can order/buy odd-sized poster frames on the cheap? I've been using plain black 1" frames with plastic fronting when I can find them, and it looks great, but the size and costs are prohibitive!

Does anyone know where I can find vintage maps of the moon or the night sky?

If you're feeling a tad more spendy, check out Swann Galleries in NYC - they are *the* auction house for vintage posters. Plus Nicho Lowry (who you've probably seen on Antiques Roadshow) is adorbs.

The best poster shop I've ever been to is Galerie Un Deux Trois in Geneva, great quality, all original and they have a pretty great web site as well.

http://www.gal-123.com/en/home.html

Here is a photo of my favorite poster purchase from them:

http://www.flickr.com/photos/29500172@N08/5488482829/in/set-72157608395431022/

If you're downloading them, they're not vintage. Miscellaneous Man sells real vintage posters. I mean, reproductions are fine, but don't kid yourself that they have any actual value.

I purchased a poster from Miscellaneous Man,
Wonderful experience, great poster, had a great conversation with the Miscellaneous Man himself, One of the only poster and paper dealers that I have spoken to that still has authentic passion for the material.
The value in reproductions is that if the studio that does the work is good enough, if the reproductions are made from a high definition and high resolution capture of the original material someone can have an entire art collection for $5000 instead of $500,000.
Collecting valuable original art is for the upper tier of the American ruling class, high end reproductions are for people with jobs who like the imagery, who simply want to decorate and don't want a bad print of a low resolution jpeg from the web, its the way of the future.
and if it gets damaged it can be reprinted for you.

Three Potato Four Shop (threepotatofourshop.com) is also a great source for labeled items. Not necessarily posters, but vintage labels and signs.

My favorites have always come from estate sales... i actually work for a fantastic estate sale locating website that will send you emails when a sale in your area lists posters (or whatever else you happen to like!)

For some rarities which might only be found within the Library of Congress, photo reproductions are an option:
http://www.loc.gov/duplicationservices/photo.html

For instance, these awesome WPA posters:
http://www.loc.gov/pictures/search/?st=grid&co=wpapos

We've gotten some great posters from the London Transport Museum shop: http://www.ltmuseumshop.co.uk/

Fantastic artworks in lots of different styles. Here's one of my favorites that hangs in our hallway: http://www.ltmuseumshop.co.uk/LTM/Posters/Historical/product/Gardening-by-Underground.html

When I was in Cincinnati a few years ago, I found a place called Vintage Poster Bank (http://www.vintageposterbank.com) in downtown. The guy who owns the shop is really nice and passionate about his collection.

For old maps visit Historic Urban Plans.

Super great collection.

http://www.historicurbanplans.com/

With so many copies of vintage posters out there, if it's vintage you want, here are a few pointers:
Vintage posters should show some history on them: often they were folded, so some crease lines or folds may be evident (even if the posters are mounted on linen);
I often say that vintage posters show their age in the same way a woman of a certain age might (I am one, so I feel this liberty is allowed!) - small discolourations, hairline cracks, etc. - these are what makes vintage posters vintage;
Occasionally you will see a tax stamp on vintage posters - the French, Italian and German governments levied a tax on advertisers, so some posters will show this (copies never do - they somehow haven't thought to add them - yet!);
Most dealers and auction houses sell vintage posters which are mounted on linen - the linen doesn't add any value to the poster but it does provide a solid backing for fragile and vintage ephemera as well as a strong base when framing.
The best way to tell if a poster is vintage or not is to deal with reputable dealers and auction houses. Those of us who have been in the business for a requisite number of years, and who have the credentials are part of an Association called the International Vintage Poster Dealers Association. There is a website where reputable dealers are listed. Like me!

http://www.laffichiste.com/

Finally, if you're unsure, or the price is too good to be true, listen to your inner radar: you may not be an expert, but that's what inner radar is for!