I don't know if you noticed, but the image I posted the other night comes from a new site called OccuPrint that is cataloging posters from Occupy sites around the world. So far I wouldn't say the collection is comprehensive, but it does seem to be growing, and it's nice that they offer the pdf downloads. (Warning to idle clickers, clicking through to the artists' sites can suck up an afternoon unexpectedly.)
If you've already been hunting around for posters for the sake of posterity (or because you're a hoarder by nature and convince yourself you're a collector of anything you have more than two of... ahem), you've run into the "Occupy Posters" Tumblr. Occupy Together has a nice set. And Occupy Design has a collection of visuals that includes posters but also contains symbols and other graphics.
The Occupy Wall Street main site is an amazing archive in itself, though scrolling backward is slow going as the media takes time to load. There are a lot of great visuals (and by the way, the logo for the Food Working Group is beautiful -see also, here) but if you're looking for downloadable posters you may be disappointed. I did find these links, here and here, of original NYC General Assembly announcements of the September 17th gathering, which I initially found in this image round-up (which I think was published on September 17). I recognize the "calling all artists" one, but what I don't see is a high quality, downloadable version of the poster for that day that you really want.
The poster you really want if you're building an OWS poster collection is the image of the ballerina on the Wall Street bull sculpture. That actually belongs to AdBusters, which does have an OWS poster download section on its site, but not that particular image, as far as I can find. The highest resolution version I find (and let me know if you know a better source), is the pop-up on this Epoch Times article.
I suspect there is a lot of great art on the many OWS affiliate sites. If your local has something good or if you know of other good sources, please share them with us.