Suksawat Lumber in CM... great place for variety and reasonable prices.
I recently bought two 1" X 8" X 3 m. boards for 1,180 baht.
It was absolutely primo - not a knot or bad grain in it. They told me it was coming in from Laos, hence the low price. I was expecting to pay between 2,000 and 3,000 for wood of that quality.
Incidentally, I buy most of my lumber from local dealers who buy old teak houses and resell the lumber.
Good luck with your projects.