i can't really think of anything in the arboretum area...
i live in northwest austin and la tapatia is my usual hole in the wall mexican place.
they do sell barbacoa to go, by the pound.
they are at 183 and anderson mill, in the same shopping center as moonie's.
if you do go, make sure to request some of the green sauce.
it is not a tomatillo-- more like a green chile aioli.