If no other tips come up, I will try the shops along Charoen Muang Road, but in case anyone has a specific shop to recommend for this, I'd be grateful for the info.

Going to use it to improve the protective coating of a hardwood pavilion. Preferably a dark brown variety.

Not hard to find. Noppadon Panit, the big Home Mart shop on the south side of the Super Hwy between the road to Doi Saket and the road to Mae Cho is where I have gotten it but have seen it at Home Pro, Suksuwat lumber yard and others. Noppadon may have a better selection for the color you want. Good luck, my experience is that it is good in unexposed areas but lasts too few years in some highly exposed areas. A friend uses it on a deck and has to redo it every year or two and my wood gate is the same. Wish I knew of something better for both of us.