Antique posters of any subject matter are pretty collectable right now. It can be very very hard to distinguish between authentic and fakes (dealers get fooled too). You can contact any good art gallery or deal and they'll have sources. BUT be prepared to pay.....I might lean toward reproductions myself for the value.
I know in Chicago there's a gallery that specificly deals in antique posters, I think their on North Ave. (if that's any help....). Breadster might know their name.