Ok, first you have to go to a reputable pharmacy or any grocery store with the following names (they have their own drugstores in there): Aurrera or Gigante or any big grocery store or like I said go to a reputable drugstore (Farmacia del Ahorro or any that seems reputable). You should go in and tell them what you want and they should NOT ask you for a prescription. Don’t go in wearing tank tops or anything because since you’re American they probably will ask you for a prescription and send you to a doctor to make a little extra cash. Just try to look “normal”. Now, there’s nothing to worry about like in the States, the only problem I see is when you try to bring them back!
I hope this helps and anyone going to Mexico.
Peace.
k (special Mexican correspondant! hehe)