I know that Tesla has discontinued the 6-30 adaptor (240V, 30A, 3 pins). Where could I find a used one?
Maybe you are thinking of the Tesla NEMA 10-30 adapter (Tesla Gear Shop NEMA 10-30) ? Seems to still be available.
They also used to make a 14-30. I can't understand why they stopped making that one. It's not even legal to install new 10-30's. They must have made a bunch of each and people actually wanted the 14-30's but not the 10-30's, so those remain in stock. Hopefully they will change to a completely different adapter-UMC interface that is more robust.