My favorite is one of the posters for Sanjuro. Everything's so cool about that one.
I haven't found too many others that I like actually. And, I've in fact found some that I really think look silly. Like, the one for Seven Samurai
I also really like the cover of the criterion dvd of Stray Dog. I think its way better looking than the original Stray Dog posters, and I so wish they had made it into a poster.
And of course, where would be the best place to buy them?