Just go down to the pharmacy in a department store or supermarket and ask for it. You don't need a prescription, but you'll need to show your ID to get it. (It's like sudafed, some people threw a bitch fit about how easy it was to buy meth a couple years ago, so they put some bullshit system in to pretend they were doing something to control things.)
The smallest box of meth you can buy is around $50, though, and they usually don't accept oral sex as payment at pharmacies, so you gotta have da cash.