http://www.internationalposter.com/Here you will find a fine catalogue, with particular strengths in Italian, Swiss and Soviet posters, along with a really valuable introduction to the subject as a whole.
SPECIAL FEATURESThe excellent article about early posters and poster collecting is found in the very small print at the bottom of the homepage, under
Intro to Posters. Also most informative is the article
A Key to Poster Value. This lays out very clearly the factors that enhance a poster's value, as well as those that reduce it, such as fading, folding or dry-mounting.
Poster Books leads to an excellent list, probably the most comprehensive you will find, and helpfully sorted under nationalities or headings such as War and Propaganda, Art Nouveau, Art Deco, Travel and so on.
For good, solid information about posters from all corners of the world, this is where to start – a really professional and elegant website. 
