On West Vernor, near Livernois. Also east of it is a Spartan or IGA called “Ryan’s” that’s a pretty good size. On Bagley east of I-75 is “La Colmena” also called “The Honeybee.” They did a huge, fancy remodel a couple of years ago, and is a good contender for El Supermercado. I seem to recall seeing another huge Mexican supermarket on Michigan Ave., southeast corner of Michigan and Livernois if I remember correctly.
Also, the Mexican food in Mexicantown gets better the futher you get from the Bagley/I-75 area (although I do like Los Galanes). Try “Mi Pueblo” – take Vernor west from I-75, follow it as it becomes Dix, and keep looking north. You won’t miss it. It’s better Mexican food than I can get in my neighborhood here in Mexico City!