Methyphenidate is a Schedule II controlled substance, the same as Cocaine, PCP, and Opiates. Most AS are Schedule III or IV. If you get busted messign with this stuff...you are f*d
If you found that it really did help you (and I have been on it for 7 years as an adult) then you need to get your primary care physician to refer you to a neuropsych for evaluation. They then are likely to send you to a special testing facility for a 1/2 day of tests - some insurance companies cover it, some don't. Then they will give you a scrip if you are diagnosed as ADD or ADHD.
Chances are your doctor is as likely to write you a prescription for this stuff as he is Oxycontin. In Mexico, I would suspect that it would be as expensive as hell, likely bogus, and if you got caught with any quantity you're screwed.
Mods, wouldn't this be considered an overt source request?