An excellent website that has a huge gallery, organized categorically as well as by time period. Some posters are for sale.

From the Maopost website:

Maopost is a site dedicated to China propaganda posters. Created by two enthusiastic collectors, Pierre Lavigne and Pierre Budestschu, it has recently been enhanced with a unique concept: the site commissions personalized oil paintings in which a real person's portrait is seamlessly incorporated into an otherwise faithful reproduction of a classic propaganda poster. Other projects, which endow the propaganda imagery with a humorous and creative twist, are in the pipeline.