Thanks for all of the quick replys!
Longbox: The wide part of the shim is in the back. All 4 tires are the same size, I used to have taller tires in the back. It seems to me that the caster should have increased, but it doesn't feel that way.
Grigg: You might have to go into business making those shims! They look pretty thick, what degree are they?

I'll try NAPA again in person. Hopefully they still can get them, or I'll try the special size order place dave posted. Does anyone know what size bolt hole I should get? Can I stack shims, or do I need to get one the correct thickness?