I can't imagine you will find it anywhere in Thailand because everyone here knows that it would never be in demand. In fact, I can't think of any product that would be more difficult to sell in Thailand.
Thais go to the extreme to avoid the sun, not because of the damage it can supposedly do, but because they don't want to have dark skin. On the other hand, the majority of tourists who come here with the intention of getting a tan, want to to get their tan naturally, from being out in the sun, just like Mother Nature intended.
Sorry it's not the answer you were hoping for, but if you want a tan in Thailand, you will just have to take a chance and spend some time in the sun.

Don't try to impress me with your manner of dress cos a monkey himself is a monkey no less - cold fact