I just came back from two weeks touring Thailand. No postcards to be found anywhere. None. Lots of Buddha and elephant souvenirs though... Not sure what to make of that, but I guess they just weren't selling anymore?
In the last 5 or so trips (Europe and Africa), I was able to both find and send postcards from all of the countries. So they are definitely out there, and it is a nice way to put a smile on someone's face, especially since they are so rarely sent and received.
just got back from there - same. They had various novelty postcards, e.g. build a bridge or elephant out of them - but no picture postcards from the region.