What visa do I need to do a TEFL course?
It doesn’t matter. It could be a tourist visa or a non-immigrant visa. You just need a visa that allows you to stay legally in Thailand for the duration of the actual TEFL course. On the other hand, once you have finished the TEFL course, you’ll possibly be seeking teaching work in Thailand, so the non-immigrant B visa would be the best visa to go for. If you are planning to do the TEFL course in Thailand and then go and look for work in another country, then you obviously follow the visa rules of your destination country.