Is there any auto store I can buy pinion shims at or do I have to mailorder them?

 

Auto parts store probly will not have them in store might be able to order them in.
I would call around to your local auto stores.
If your in a jam you could buy some shim stock and make them. just trace them out of your old shim and stone them to get rid of the burrs.
When I got my gears I ordered a bearing kit that came with shims.