Thanks for the info. I am very familiar with Garmin as I used it on my boat in the U.S. However, I have researched their info and I find that there is an Asian version which has a base map for this part of the world. Their sight is not too helpful however in finding a dealer here. They do list one in BKK but they do not have a website. I'll try next time I am there.
Magellan seems to be the best unit feature for feature. They also have a sytem which is designed for use in this part of the world. Again I have not had any luck in finding a dealer. The units, both Garmin and Magellan, sold out of Australia are supposed to be configured for use here. I am still looking.
I will check with the source you mention in Chiang Mai.