i get my carne asada from a store on the corner of osborn and 19th ave. i forget what the name of it is. i think it is phoenix farms market or something like that. anyways, they have really tasty meat, and they will grill it for you for free. i think the kind we get is diesmillo and it runs about 2.50 per lb. it went up a little. now it isnt the cleanest place but the flavor makes up for the older interior. go to the meat counter not the restaurant
Carniceria Sonora is really great. It isn't huge, but they have the best chips I've ever purchased. Seriously. They also have delicious homemade salsas. I haven't tried their meat counter, but have been told by coworkers native to Mexico that it's where they shop for their carne asada meat. In fact, I started going there on their recommendation.
3135 South 48th Street, Tempe, AZ 85282-3182
Tough question to answer because you added in "authentic." Phoenix Ranch Market is definitely among the top. I only hesitate because I've only been in a couple of Mexican markets - among many, many - that are as clean and well merchadised as PRM. There's probably some small carniceria somewhere that knocks it out of the park but I haven't found it.