Several places I use are:
DDM (Dave's Motors) - davesmotors.com
Scooterparts4Less.com
PartsForScooters.com
ebay.com
Basically, anything that will fit on a 47cc engine, you can use.
Now, if you're looking into one of the big bore kits, I would suggest shooting a PM to Boosted306 before purchasing. Sam is our resident engine man, and he can make these little two-strokers sit up and bark. Friendly guy, very helpful, and the quality of work he does on cylinder heads and pistons is second to none.
gfracerx (also here on the forums) sells IP2 race pipes, fold-up footpegs, and assorted other parts. A good place to check for parts and bikes is right here in the
classifieds section, as many members have stuff they don't need/want, and are willing to sell it.