The official website
http://www.ilpalermocalcio.it/en/home/index.jsp used to have a shop-online page but now it's gone. I think they're doing some maintenance for the beginning of the new season (the first official match is scheduled for august 23rd) so that they can include all the new stuff (new shirts, scarves, flags, hats, sweatshirts, jackets etc...), I know for sure that they deliver anywhere including USA so I guess you'll just have to wait a couple of weeks or so and that page will (hopefully) come back up
I am not sure about posters or books, all the most important books are in italian anyway, I doubt that they are avaliable from the official website, maybe you can try to have a look at amazon.com
.. i am afraid I can't really help you there but i'll try to get you some information
