You should be able to use any roughly circular flat object... I used small screws that went though the servo horn's limbs and into undersized holes in plastic k*nex wheels for my $50-equivalent bot. How big a wheel doesn't really matter, within reason. If it's too fast (aka too big a wheel) you just slow down the servos... As for source of screws... you do have a collection of random screws from taking apart dead electronics and toys, right? =D