Nice place to buy art prints/posters in Warsaw?

Would anyone be kind enough to recommend a place with a good collection of relatively cheap and cheerful posters to jazz up my little slice of the city?

On the Rynek is a shop, but not exactly cheap.
Also I think the Poster museum sells them...

Dziękuja bardzo

I used this site when I lived abroad. They have great sales ($9.99 on most classic posters) and they ship worldwide.

Thanks Isa. I'd been looking at the English version of that site.

Not sure if this is the shop mentioned above, but here's a link I found: poster.com.pl