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

http://web.me.com/kongm/Kong_Mangkornkarn/About_Me.html
The pages of Kong Mangkornkarn

http://www.poss.ws/
The outstanding pages about the Hmong and their music by Nicholas Poss

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

SHOPS:

http://www.ttcrafts.co.th/cgi-ttcrafts/producttype.cgi?producttype=Musical%20Instrument/
An online shop selling musical instruments mainly of the ethnic minorities

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

http://www.freewebs.com/ruenroeng/index.htm
A nice shop page showing how the xylophones and drums are buildt 

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