Believe it is called Stillnox here, and it is in the most severely restricted category: category P2 - Psychotropic Substance Schedule 2 , hence only hospitals can purchase directly from FDA - Ministry of Public Health, and not available in street-side pharmacies.
I doubt any doctor here will script this for you, maybe some Doc may give you a few tabs to tide you over? Maybe stock up on some melatonin?
You can get more detail on Thaivisa, simply use
drugname site:thaivisa.com
try stillnox, zolpidem, ambien
example: zolpidem site:thaivisa.com