What 20V do you have, Silvertop or Blacktop?
What are your goals for the motor? Stock rebuild, mildly tuned or all out race built?
For majority of your service and rebuild parts(pistons, rings, gaskets, oil pump, timing belts, etc etc), source the Toyota part numbers and then hit up
www.amayama.com for the parts. They are cheap and pretty quick at getting the parts (no 4-6 weeks from Japan BS).
Aftermarket parts are all dependant on what your goals and budget it.
Several companies make aftermarket pistons.
Kelford Cams NZ is probably one of the better places for 20V camshafts and springs.
SQ Engineering has a bunch of good auxilary parts (COP plates, velocity stacks).