I had a friend bring me back kosher, parve fish sauce from Israel - not vegetarian. I also get people to carry back anchovy paste, bean thread, and other products from there. Almost all anchovies are marked kosher, so making your own paste or fish sauce is also an option. The vegetarian suggestions by foodfighter and innoabrd both sound good.

It appears I omitted the adjective "vegetarian" in Vietnamese vegetarian fish sauce. Important detail, in that nuoc mam an chay is purportedly vegetarian. Although I don't know if it's pareve.

I agree with innoabrd, anchovies.

Nuoc Cham is a typical dipping sauce or dressing that uses Fish Sauce as an ingredient, or a fish sauce subsitute if you are vegetarian. It isn't a substitute for the fish sauce itself.

following on pierino--I found this link:

http://www.phamfatale.com/id_363/title_Nuoc-Cham-Vietnamese-Vegetarian-Mock-Fish-Sauce/

4 tablespoons soy sauce or vegetarian fish sauce
3 tablespoons rice vinegar
5 tablespoons boiling water
4 tablespoons brown sugar
2 teaspoons lime juice with pulp, freshly squeezed
4 teaspoon garlic, finely minced
2 teaspoons red Thai chili pepper, finely chopped, to taste

One of my food references suggests combining soy sauce, rice vinegar, powdered dulse (seaweed) and rice vinegar and fermented tofu. I don't especially endorse this, as I happen to like fish sauce. There is also a Vietnamese sauce called nuoc mam an chay. I don't know what it contains as fish sauce is made from fermented anchovies.

see this foodpickle question, which provides some answers

http://www.food52.com/foodpickle/858-what-is-substitute-for-tha

Mmm. Can you get kosher anchovies? Have to admit that soy and lemon don't quite do it for me, but I wonder if you could try simmering some anchovies in water and then straining. It is, after all, a fish (usually anchovy, as I recall) extract.

You could also try some Worcestershire sauce and a little soy sauce, the Worcestershire has some fish in it and the soy sauce is salty.

thank you so much, foodfighter! I needed a kosher substitute because most fish sauce isn't marked kosher, and kosher is a) nearly impossible to find and b) really, really expensive.

2 parts soy sauce, and 1 part lime juice. Are you asking because you don't have fish sauce, or you need to cater to an allergy? There are different types of fish sauce, so with and without shellfish.