Where's the best place in the U.S. to find original Buster Keaton
movie posters? What are the dimensions and styles of posters from
that era, and what are the price ranges today?

I know very little about posters, so I can't offer anything useful in
that line, but if you're a Buster Keaton fan, be sure to check out
this site. You may want to become a Damfino, if you're not one
already!

http://www.busterkeaton.com/

Howdy again.

A friend who has a small collection of movie posters recommends a
Canadian firm called "The Hollywood Canteen" as a good source for
original posters. You might want to drop them an email:

http://www.hcanteen.com/