Go to a Mexican meat market or supermarket and ask for "arrachera", they should have it. I don't know about other parts of the state but in the Valley the La Michoacana stores I have been in all have real Mexican cut arracheras.
I found outside skirt steak randomly at the Nutty Brown HEB (Austin/Dripping Springs) this morning. It feels like I just scored a bottle of Pappy's because it's so hard to find.