What is the best way to find a good Thai teacher for private lessons: iTalki, HelloTalk, or something else?

It all depends on the level of Thai you want to learn. The licensed Thai language schools teach formal Thai almost exclusively. Private one-on-one teachers will mostly teach colloquial Thai, and even down to the level of what's referred to as bar girl Thai.

It depends on why you want to learn Thai as to what level of language training you should look for.

The best are the university language professors that tutor on the side for extra money. They do teach Thai in the universities, especially in the teacher programs.

Preply. My friend is an awesome teacher. Can recommend.