I just posted this in another thread, "Thumb Rest Best Practices."
In case any lefties want a thumb rest to add to a Fender Jazz or Precision bass, here’s a company that makes some for LH models (for thumb rest that attach to a pick guard): The Thumb Spot (page “For Lefties”). This company also offers two “universal” type thumb rails (“UniRail” and “5thRail”) on the same page, which attach with adhesive, for other basses.
Zero Mod also makes thumb rests that attach using screws already there for the pick guard, and maybe pickups in one case (check “Models” and “Special Order”). Some look like they’d work okay on a left hand bass, depending upon the brand and model, and many are for brands other than Fender. I suppose they might make a specific LH thumb rest by special order, but you’d need to ask them. Zero Mod items seem more expensive than those at Thumb Spot, though. You be the judge.
Just FYI! 