I managed to get one from a BMW but only because a mate had a spare handy. When the other one failed, I got one from Mr T. It was expensive, but I didn't have much option at the time. You could get one from Karl @ Landcruiser parts, or Garry at 80 Breaker. Postage would be next to nothing. Buy a pair, the other side is just as likely to leak soon.
It's not something you can fix, I had mine apart, but you effectively break them opening them up. It's also unique for the purpose, it's not a valve as such, but as you described, a pressure sensitive check valve to stop the siphon.