multiple colours and i think you can blend it..

http://www.toysdirect.com/products.php?category_id=121

erm dont know any US sites sorry i live in england.

you could also try play-doh which similar but it sets in time, where as plasticine doesnt.

http://www.hasbro.com/playdoh/

also stick "plasticine animation" and there are a lot of helpful sites about how to get started plus some youtube vids

this ones freaky
http://www.youtube.com/watch?v=Gj7auB0-su0