You won't be able to fit 6 shims in the diff during initial assembly. The gear mesh will get loose after you run it enough (happened after second gallon 4 me). The rear diff will skip once that happens. At that point, you'll need the extra shims to get the mesh back into spec.

U could use any shims available at your LHS (as long as they're the right size). Here's a pic of the shims I used (non HPI specific):