I wouldnt think you would be able to get Ritalin (methylphenidate) or adderall (amphetamine salts) in Thailand as they are both stimulant based medications..even though they are low dose. After all the Thai govt is set on a morale crusade to eradicate stimulant like drugs and drugs of abuse from Thailand.
Atomoxetine may be available though as it is not stimulant based..instead its an SNRI with very good efficacy in ADHD. Id reckon that you could get this in Thailand very easily.
