Blatantly sold from a bar in the Loi Kroh area of Chiang Mai and customers come from other bars to buy the balloons and take back to their waterhole.
I tried it once, just gave me a headache. A friend of mine who works at the bar who sells them has at least 4 large balloons everynight costing 100 baht each. Some nights more drunk on the balloons. Combined with alcohol not good. But she did learn lesson the other night. Caught riding motorbike drunk. Cost her several hours in a cell, a trip to the courts and 9500 fines.