Check this thread which sadly turned into a car thread -
http://www.allposterforum.com/index.php/topic,6763.0.htmlI went there in August, great guy to meet.
The link Charlie posted is correct, it is called "Classic Movie Poster" but then good luck to find it, it took two days to find it (because of language barrier and the shop is totally unknown).
Well, first of all, if you are in the hurry, then Siam is the place to visit (small tip, do MBK mall or Bangkok Art Center (BACC) in the morning time because shops in Siam usually open afternoon time)... if you go there in the morning, it is kind of dead.
The address is very very correct - look for Lido Theatre at Siam Square Soi 2... the Lido shopping mall is an old three storey building full of small stalls selling clothes/vernishes inside, and surrounded by stalls as well. The shop is located in a tiny corridor linking main street to one of these external corridors surrounding Lido.
Good luck because it is very hard to find; I asked to 50 persons and none knew about it. As for me, I bumped into a tiny art gallery on second floor of Lido Theater called ArtGorillas -
http://www.artgorillas.com/ - and the lady, who speak English and is art oriented, knew the place. (guess what I did all corridors of the mall except this one, damn)
The guy is called Santi (note the address there is not the address of the shop but his personal mail)