Originally Posted by
Diplomatico
7-11 and Family Mart no longer stock Thai SIM cards since the ruling military junta started requiring Thai ID or passport for registration when purchasing one.
Wasn't aware of this when I went to Chiang Mai last year. Asked at a couple of 7-11's, which would all say "sold out" and to keep walking towards some other branch down the road. Turns out there was one 7-11 that would have stock and do the passport registration thing.
Lesson learned was to def take care of it at the airport, which is what OP should do