I ordered the posters from posterbrain.com, they were the most reasonable cost wise in the size that I needed. I just got them in and they look great.
I ordered the foamcore from Joanns Fabric. They cost me around $12 or so. I also got the semi adhesive removable dots, and the easel, from there as well.
I saw the idea on tk and the girl that did it actually wrote a blog with instructions! Here's the link if you're interested.
http://sites.google.com/site/januarybride/diy