Hi there,
meanwhile I found some contour lines in osm format on the web (http://geoweb.hft-stuttgart.de/opendtm.html). They are not perfect, but still very useful.
The whole map of Thailand with those contour lines measures more than 200 MB - hence I cannot publish that (limit of my homepage is some 100 MB). But I updated the map without contour lines (http://www.berniesmaps.de/Thailand.img).
But the Andaman coast (7-11 N, 98-100 E) is only 12 MB, and it is available here:
http://www.berniesmaps.de/ThailandAndaman.img
Have a lot of fun!
Bernhard