By jove, I might have something here! I'm on a Vietnamese cooking kick, and it's hard to get much of anywhere in that cuisine without a fish sauce sub. To my knowledge, there isn't a premade one on the market. I was aiming for something to fill the clear-pungent-saltiness of fish sauce without importing that nasty (IMHO) fish taste. Soy sauce just hasn't satisfied me as a substitute.
Here's what I did (very approximate--I just kept adding and tasting until I was happy):
2 Tbsp. seasoned rice vinegar small splash (1 tsp?) white vinegar enough water to make about 1/3 cup somewhere between 1/4 tsp and 1/2 tsp. sea salt--it should be noticeably salty very small pinch onion powder--you just want a little hint