Bro,
it's been 5 or so yrs since i've been there...most of the time if you go to a pharmacia you will be offered test cyp, deca, and sust tabs..the last time i went was Matamoros, crossing at brownsville, i happened to walk into a shop that sold leather and what not. the guy who was waiting on me looked me over and asked if i was interested in "get big drugs", anyway long story short, he drove me to a pharmacy deep in the city, went in the back door, and was offered 10ml cyp and 50ml deca.
you may not feel safe doing this but i had been across the border a few times.
There is a post here about a "mexican plan" click on it, I posted at the end with how i usually brought gear back across..
good luck, JC