I have done some research.
Poppers, as well as other substances such as glue, lacquer, and thinner, are covered by the The Emergency Decree on Prevention against Abuse of Volatile Substances of B.E 2533 (1990).
You can find the law in its entirety here:
http://thailaws.com/law/t_laws/tlaw15028.pdf
To cut to the chase:
Use or inducing another to use carries a potential jail sentence of up to 2 years and a fine of 20,000 baht;
Importation or sale raises the stakes to 3 years / 30,000 baht.
Something to keep in mind!