I've done some research, and came up with Yoskarn. Seems to have a very good reputation. Unfortunately, when I sent them an e-mail with questions about fillers, it turned out they are double as exepensive as in my own country (western northern europe).
To compare I also asked prices at Dr Prinya, they charge half of what Yoskarn charges. Which is only slighty less than what I pay in my country. So I decided not to have any botox or fillers in Thailand.