I thought, maybe Thais don't drink Decaf? I asked my wife, and her first reaction was confusion, so I explained what it was and she said no they don't. So that might be the reason. I'm in the UK until Sunday, but when I'm back and in a supermarket I'll see if it's available there as I'm not sure if I've seen it.
I would have thought in tourist areas it should be easier to find. I seem to remember you are in Phucket, although I might be wrong.