I spent about 6 months in thailand last year, studied thai for a few months, speak a basic level of the language, have heaps of thai friends, and took just about every drug imaginable while in thailand.
Would I reccomend it? no, but I was using alot of benzos at the time and drinking most nights, so my judgement wasn't exactly all there. When I went to thailand I had the attitude that I wouldnt touch any drugs, but like alot of things over there, you become complacent.
I made plently of friends with uni students, young groups that just took pills/coke as part of a night out, and smoked weed at their homes, much like my own groups of friends back in australia. Most of this was in chiang mai, amongst middle class young thais. I encountered a surprisingly big coke scene there, and could score coke much much easier than I could in Aus.
I bought it from uni students, dj's at clubs who I'd become friends with, one guy sold it straight out of his travel agency, he actually had a filing cabinet full of just about any drug you wanted, next to a filing cabinet with the legit travel packages. I also met a few long term expats who had legit businesses, seemed quite well off, and also sold and did loads of coke.
I've also met plenty of guesthouse owners who sell hash chocolate/ weed / mushies straight over the counter, some even put it on your bill. This particularly happens on all the islands, but even in bkk/chiang mai alot of guest house owners sell it to the endless supply of backpackers coming through their doors.
Yaba/meth is looked down upon by pretty much everyone over there, and while is widely available, is mainly used by lower class druggos, bargirls, addicts etc. Even the biggest cokehead would look down at someone who even touched yaba in my experience. It's also heavily controlled by the police. Ive seen police deal it at clubs, thai middle men buy it from the police (uniformed and undercover) to then sell to the customers, in full view of anyone, in nightclub districts. Just about any bargirl would know where to score drugs, but theres 0% chance you could trust them not fucking you over. I honestly prefer yaba over coke, its much much cheaper, but the people you have to associate with is what put me off using it again.
Thailand is a weird place, and I'm not gonna pretend I understand it any better than any other foreigner who goes there. But the system is very different, and corrupt to it's core. Often the people who get caught are stupid/unlucky or setup. But for all the scare stories you here, I can tell you drug use is just as widespread as anywhere else.
Would I do drugs there again? I'd like to say no, but thailand has a strange effect on people, and the more time you spend there the more complacent you become with regards to alot of things...
So yeah I wouldnt reccomend it, but then I wouldnt really reccomend doing drugs in the first place, it's a huge risk, and I just assumed having a few grand in the bank would give me enough to buy my way out if I needed to. But after reading psytaco's' story apparently that doesnt always work either.
Note all this is just my experience, I went there with the intention of living for a year or two, had a job I could do online from anywhere, made friends with alot of locals, and didnt touch any drugs for the first few months until I made friends with other users I considered trustworthy (well enough as you can trust someone in a country like thailand). It was actually my spiralling benzo habit that made me come back home, but kinda glad I did looking back at the stupid shit I was doing over there.