Tilapia is typically farmed and is unusual in that it is a vegetarian fish (excellent for people worried about sustainable farming practices!). It has very little flavor on it's own, however, which makes it a good choice for highly seasoned dishes like tacos, where it basically acts as a protein carrier of flavors. This makes it a pretty inexpensive (relatively speaking) choice at the fish counter.
Specifically for fish tacos I've had both cod and salmon. Salmon wouldn't be a good choice for this particular recipe because it is already quite naturally oily. Thin cod fillets would be a good option, as would thin sliced haddock, plaice, flounder or other flaky, smaller white fish. Orange roughy would be too dense, as would chilean sea bass (and both are quite a bit more expensive and not the best environmentally-conscious choices to make) I think that catfish would be too oily too... Trout and sea basses might also be a bit oily for frying, but it would depend on the particular species
In short, my recommendation is to ask your fishmonger for an inexpensive flaky, non-oily, white fish for suitable for frying.