It sounds like what you want is http://www.morethanposters.com/ They have a good collection of many different styles. Soviet, WPA, British World War, Spanish Civil War, even WW2 French and Belgian. Most posters cost $19.99, but for how obscure some of these are it’s actually a good deal, and it’s free shipping to the US and Canada. You may also like their collection of vintage TV and movie prints, like this King Kong.
I also use art.com. They have a wide selection, they’re very big so the service is reliable, and the prices aren’t bad. But don’t ever pay full price there. Sign up for the email alerts and they’ll send you coupon codes. I get those things all the time, so 30% or Free Shipping isn’t uncommon. Some of mine include vintage Guinness ad, vintage Egypt travel poster, To Hell With Hitler poster, and so on.
You’re actually in luck, because they have a January clearance sale that ends tonight (Friday). Up to 75% off on clearance stuff plus 20% off side-wide. Use code EAGLES. I’d totally hit that up, but I’ve got enough posters and I’m about to move anyway. (Upon Preview: Actually, now that I’m looking at the selection…)
You can get Tiananmen Square Tank Man for $10 (incl. shipping) off Amazon link. In fact, Art.com is on Amazon Marketplace and MoreThanPosters just opened their shop on there as well.
In a December holidays gifting thread someone linked to northernsun.com, which has some cool posters. My favorite is this Black Death European Tour 1347. Only 8 bones.
On another note, I don’t know what the terms of your apartment lease and the size of your security deposit are, but on a cost/effect ratio one of the cheaper ways to decorate a wall is paint it. You can do an accent wall (where one wall in a room is painted a certain color), or you can paint a design. Either draw and brush, like this cool tree, or take out a stencil and spraypaint the fucker. Directions on that here.