2 of the biggest suppliers of plasticard is Evergreen and Plastruct. You can find the links to their websites
here!
It comes in all sizes, shapes, and even color!
I have heard that a good cheap alternative is using for sale signs and such found in hardware stores. I haven't tried using them so to be honest I don't know how they compare but it might be something to look into.
As for where to purchase it... as already stated a good hobby store should carry it. The best source I have found is model railroad hobby stores. I am sure you can order it online as well.
Good luck in your search.
Cheers,
-Mike