I have lived in Thailand for years, you won't get AS in Kho San Rd...too many tourists and cops. And you definitely won't get it at boots I can assure you. Avoid pharmacies in large department stores. Go to Suckhumvit rd (it's where all the good looking chicks and bars are anyway) start at the beginning (Soi 2) and walk along any side for 10 to 30 minutes, stopping at all the pharmacies along the way...you'll eventually find someone to sell them to you.