Blastersmiths have lots of demolisher parts, including motor covers and battery doors as well as more useful things like rewire kits and mod motors.
Cosmetic parts like the ones on Jodocast etc are not as common over here. We tend to make stuff from scratch.
The ones on BlastAways latest vid are from 3D Printed Solid. Their catalogue looks pretty cool but I haven't actually had any so can't attest to their quality. Looks okay from Drac's and BlastAway's vids.