Why not get an artist to paint some nice letters/numbers on? Brooklyn is loaded with people who could do a good job.

Oh, I love KASSGOOD's comment. That's a great idea. You could have the artist paint on a plaque of some sort that could be removed if/when the door ever needed to be repainted.

I applaud the artist idea, too. A removable plaque would be elegant and feel more custom. I would also recommend asking the artist to seal the paint job well, to make the plaques easy to wipe down occasionally. Please share pictures with us after your new numbers are up!

Judging by the stairwell, it is hard to tell the style of the building. I recommend painting
or doing a vinyl decal of the numbers and letters - a big super graphic for each door would be fun.

http://www.gdhta.co.uk/wordpress/wp-content/uploads/health-centre-door-numbers.jpg

Did you try hardware stores? Home Depot and Lowe's have websites too if you don't have one nearby.
Or Etsy?
Do they all have to match? You could try yard sales - if they are all real brass, they'll be nice even if they don't match.

Home Depot has a set of shockingly modern, clean, and simple house numbers in-store. Beautiful font. Around 5 bucks per digit, I think.

Home Depot.

Laser cutting is a relatively simple and cost effective way to get just what you are looking for. I know it is becoming a more readily available option through both sign shops and independent suppliers. Just make sure you use a supplier that does everything in house, otherwise you will be paying third party rates for outsourcing.

Lee Valley has some: http://www.leevalley.com/US/wood/page.aspx?p=41839&cat=1,250,43232

They're fairly inexpensive - $2.60 - 2.70 each. Apparently they're being discontinued, so you'll have to see if they have all the nunbers you need (there are no 8, 9 or zeros)

They aren't super cheap but there are always deals online. Home depot has these really chic architectural numbers. These are in an antiqued, satiny copper which i think always looks great, especially with gray.

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=202344999&storeId=10051&langId=-1&catalogId=10053&MERCH=REC%2d%5f%2dNavPLPHorizontal1%2d3%2d%5f%2dNA%2d%5f%2d202344999%2d%5f%2dN

There are quite a few nice modern and simple choices at home depot for even less than this style, averaging $4 a pop.

Good Luck!

these arent cheap but are mordern
http://www.chiasso.com/store/item.aspx?DepartmentId=50&ItemId=56700

i got my house number at Lowes and love it, its mordern and it stands out from the door with kind of like a spacer, so it casts a shadow sometimes and it just looks nice... nto sure if they have letters too or not, i will have to check...but they are cheap....

The one i have is by Atlas, called Avalon.... or at least it identical to whats on this next site that ill post... but cheaper at lowes, i think its was maybe $6....
i like the paint idea too :)
http://www.knobsandhardware.com/contemporary-modern-house-numbers_689_s3.html

oops sorry they are at home depot and i dont think they have letters :/

Sorry, that hallway looks pretty drab. Why not try to do something more bold and modern, like really big letters, either on the doors perhaps in a bright color like white or yellow, or right next to the doors. You can get someone to paint them or order stickers from Blik.com. Here's some inspiration: http://apostrophe9.tumblr.com/post/2631368642

Removed the green and maroon? Good move.

Replacing with crisp gray? To make it look like a prison?

I like the suggestions to do wall art with the numbers, or a decorative plaque, but that would depend on how much time, energy, and/or money you have.

Just go to a sign shop. That way you can pick the exact size, material, font, etc. you want instead of having to settle for what's available at a store.

See if you can find the door you're looking for here - http://www.dooremporium.com/Cat/InteriorDoors/19.aspx. I got mine last week. Interior door quality is excellent.