According to Wizard News, sales of the upcoming movie poster for Harry Potter and the Prisoner of Azkaban are pre-sales only. The poster itself hasn't even been released, so buyers beware. It may or may not be the Sirius Black poster seen on Azkaban.com:
All Harry Potter fans should be warned that the people selling these posters, mostly on eBay, are not selling actual posters at this point, they are PRE-SELLING them, which means you pay them now, and they'll send you a poster WHEN THEY COME OUT. Furthermore, these sellers are simply using the image of the Siruis Black wanted poster as an example, it has nothing to do with what you may receive if you purchase one of these pre-sales. (This text copyright © 2003. Reprinted with permission.)
With that said, we've contacted the eBay seller from our previous poster news who states he obtains his posters from a high-level source:
"I am 100% sure that any poster I sell will be 100% original and straight from Warner Brothers to the customer. The posters I sell are printed for movie theaters and are supposed to be sent straight to movie theaters but some are stashed away for me. I am 90% sure its the Prisoner image. The image is already wide-spread, so why wouldnt they use something they have already made? Warner Brothers has already been using it on the lamp-post standee for theaters.
"[Additionally] I clearly state in my auction that if they are not 100% satisfied and that if something unforseen occurs everyone who purchased the poster will receive a full refund from me. If the poster image winds up being different from the one that was shown on my auction I will email the winners and ask them if they want a full refund or the new style poster. They will get whatever they ask for with no questions asked and no problems."
Finally, keep in mind that this is a teaser poster, much like the owl post in Sorcerer's Stone and Dobby for the Chamber of Secrets. The poster featuring the characters is unlikely to show up until about a month before the movie's actual release.