I've used a couple of the suggestions mentioned (anchovy paste and oyster sauce). The try another brand thing wouldn't work for me .... ha-ha. When I first moved here there was no such thing, now we're lucky to have one on the shelves of one of the three chains in the Galena/Dubuque area. There used to be an Asian store in Dubuque, but lately any time we've gone they've been closed, so we haven't even tried in years. Maybe next time.
My biggest problem with fish sauce is almost every brand i've been able to get over the years come in large bottles, and I hate to give up that much fridge space to something I don't use very often. Then, too, I go to use it and it looks just gross and smells very, very, very strong after sitting on the shelf for a year or three. So I usually do make do with substitutes that I use more often.
Worcestershire sauce has some anchovy in it and in some things works.