There are other threads on here about this. But here you go. They sell both copies and the real deals. They are expensive sometimes but authentic if you want to spend the dough.
I have been collecting and trading original film posters for 22 years.Mostly posters from the 1930's-early 1960's.All online stores are are way overpriced.
emovieposter.com auctions is the best place..they have 3 auctions a week Tuesday,Thursday and Sunday.Every poster starts at $9.99.No buyers premium just shipping cost.And usually 500-900 posters in each auction.US and foreign,Each auction is usually a different size One Sheets,Half Sheets,Inserts,Lobby Cards,3 Sheets and 6 Sheets,Photos etc.
Heritage.com poster auctions is another but they charge a 19% buyers premium on top of your winning bid.They have poster auctions every Sunday night all starting at $9.99.And 4 major poster auction a year.Where you will find the rare stuff.