Taqueria Los Gueros is in Anaheim, not too far from the convention center. My personal favorite is
Taqueria El Zamorano, but it's in Santa Ana, which is a bit further.
Taqueria De Anda is a local chain but they are legit, and can usually be found in most neighborhoods in Orange County.
The general rules for excellent taquerias in Orange County, though, are the same for Thai food: you want to find a strip mall built in the 60's or 70's that hasn't been updated sine the early 90's. There will probably be a smoke shop or a small hardware store or a bar nearby, and it's likely that the giant lighted sign in the parking lot will no longer light up. Any taqueria that meets those criteria is worth checking out. Bonus points for a kitchen that has an old lady in the back cooking something in a giant pot. If there are a bunch of Hispanic dudes wearing white cowboy hats eating there you've hit a gold mine and need to tell me about it ASAP.