I've done this a lot... go online and download the addresses of distributors... then... and this is the big important thing that most people don't do... CALL THE SHOP IN ADVANCE and tell them what day you are coming and what you want them to have in stock. (Get a Spanish speaking friend to do it.) It is VERY hit and miss if you don't call in advance. Then ask the cabbies who speaks English... you can usually find an cabbie with some English.
Next... tip the cabbie big time and behave respectfully. Mexicans are a beautiful people.
went to cancun last year. a taxi picked us up asked us where we wanted to go? i said "PHARMACIA" he asked if i wanted sex pills (date rape drug) told him what i was looking for he took me straight to a vets office and then to a small doctors office. done in under 30 minutes. taxi cost me 8.00 tipped 50.00. easier than christmas shopping