The stuff he is asking about is legitimate meds not controlled substances and he is not asking about sources or source checking. With that being said, the laws in Mexico are different than they are in the States so you can go to Mexico and purchase prescription meds at ANY legitimate pharmacy in Mexico without a script BUT you CANNOT bring the stuff back to the States.
Now sourcing is a big no no here and that you have to take somewhere else i.e. the board mentioned above.