I have found a great website which stocks alot of import pit bike parts. These include frames, drivetrains, exhausts and alot more so definitely worth a look.
The site sells stock and performance "pit bike parts":http://www.gomeyer.com/retail/pit-bike-import-pit-bike-parts-c-709_638.html so whatever you are looking for it should definitely be on this site.
Spare site
I found a really great site with spares for pit bikes and mini motos as well as upgrades. www.motobikespares.com looking good !