I recently used some of the WPC (Wood plastic Composite) from Thai Watsadu and to me, it looks like real wood that has been painted with a glossy paint. As for door/window casings, I couldn't find any good ones, anywhere. I again used some WPC from Thai Watsadu, but had to be careful installing it because it was hollow. Finished product looked OK.
I looked all over (in Chiang Rai) and couldn't find anything better.
For a different house in Chiang Rai several years ago, I did find some colonial style door casings at a wood supplier, but they were pine. Our house is in an area that has lots of termites but so far, so good. Perhaps because we didn't use real wood anywhere else other than doors and door frames.
In the Bangkok suburbs, there used to be an area of Bang Sue that had many wood shops and I THINK they had wood for sale. You might want to try there.