I have been to both Cancun and Jamaica in the past year. I found getting meds would probably be much easier in Jamaica. I had people offering me everything on the streets in Jamaica. I didn’t buy anything at either location, I was on yoga retreats and not with the kind of people that would approve of such things.
But in Jamaica, I literally had somebody try to put a bag of crack in my hand! I handed it back to her very quickly. Very very pushy dealers in Jamaica. In Cancun, and all the other surrounding areas I never had anyone approach me to buy meds.
If you try to buy them in a touristy area, I would think that you would get charged a fortune. Your best bet would be to find a friendly native to the area that would help you. I read up on this a bit before going to Cancun/Playa Del Carmen/ other areas nearby, just to see if I would be able to get benzos. From what I read most pharmacies still ask you for a script. Humorously, I was able to buy meds off of another tourist while I was there.
Just be very very careful. There are a lot of desperate people there that would be more than willing to take your money and rip you off. Make sure that you understand the currency exchange, a lot of people get taken for a ton of money when they don’t know the correct exchange rate. Do not exchange your currency at the airport! They charge you around $100 to convert $300 into pesos at the airport.
Good luck. I would think you’d be able to find something, it’s just going to take a little bit of work.