Links:
http://www.tribalmusicasia.com/
A very good blog about the music of the "Golden Triangle" by  Victoria Vorreiter

https://www.thaisounds.com/ethnicmusic.htm
A page with many audio samples of music from various ethnic groups

https://goldentriangleinstruments.wordpress.com/
A very good blog about various instruments of the "Golden Triangle"

https://www.thenotedproject.org/
A project introducing the Karen musicians´on the Thai-Burmese border

http://www.paradisemoon.com/thai_main/Huahin/thai_instruments_intro.htm
A nice overview of Thai music written by Greg Turner

http://archive.org/details/calauem_201405_I-E_clip_Music_of_Thailand
A movie of the UCLA Music of Thailand Ensemble from 1972

http://www.hmongstudies.org/
The pages of the "Hmong Studies Internet Resource Center"

http://sonicsafarimusic.com/music/thaifolkmusic/
Audio exmaples of Thai Folk music by "Sonic Safari Music"

http://www.indigenius-recordings.com/
Recordings of the music of ethnic minorities in Thailand

SHOPS:

http://www.thaimusic.net
An online shop selling musical instruments

http://jidathaicraft.com/E3-3%20Tee.html
The "Jida Thai Craft" online store for Thai musical instruments

http://musical-instruments.novica.com
The "Novica" store in Thailand, also selling musical instruments