Yes I believe Den farmerii is native to Thailand
Actually Den farmeri is not only native to Thailand but to continental S E Asia including Indochina, Myanmar, Penisular Malaysia and even to parts of India and Nepal. I have personally collected 3 varieties of Den farmeri from Langkawi Islands and a hill in the Ulu Muda area in my own home state. Most are of the variety as shown in Jojo's photo but there are also the ones with light pinkish petals, and there are the ones with more orangish-yellow centres