From their website, "To order please email the AMS Public Awareness Office at: paoffice at ams dot org with "poster order" in the subject line."
Promote mathematical research, its communication and uses,
* encourage and promote the transmission of mathematical understanding and skills,
* support mathematical education at all levels,
* advance the status of the profession of mathematics, encouraging and facilitating full participation of all individuals,
* foster an awareness and appreciation of mathematics and its connections to other disciplines and everyday life.

You can order a free 16-month Wall Calendar for 2011-2012 along with various posters, including one with a picture from the movie, Avatar.

AMS Printable Posters