I've been to Thailand and never seen opium sold in the outside markets. Of course, it can be found on the streets and as far as I know it is illegal. Illicit drugs in Malaysia, Thailand and Singapore are taken very seriously. Whacks with bamboo sticks and life sentences are handed out like candy in these countries. Also, drug trafficking will get you a death sentence.
Anyway, I will inform on the pharmacies in Malaysia in a couple of days. I am currently staying about 4 hours away from KL.