I tried to get borax last year, but found out it is a restricted product in Thailand, due to misuse in the food trade (used to be added to noodles to make them last longer). Not available at a pharmacy, or any retail location. You may be able to buy from a chemical supplies wholesaler or on the internet. I brought my own back with me in September (if anyone wants a small quantity). However, travelling with a plastic bag full of white powder through customs like i did could be problematic...

I do not think Borax or boric acid in the soil works very well - it needs to be ingested to work- hence usually used as a bait mixed with sugar or painted on wood liberally, so if eaten by termites they will get terminal indigestion quite quickly. But it's cheap and pretty safe to use - an adult would need to consume a couple of ounces in one go to get sick.

I was told it is called 'kabor' in Thai but when i said that no-one understood, so possibly wrong. On the solubility front, Borax (sodium Borate) doesn't disolve all that easy - I found you needed hot water to dissolve much. I use it to kill ants by baiting them when they come into the house; because it takes a while to work it is shared around the colony and a good chance you will kill the queen as well. I am still seeing less ants now after stopping 2 months ago. Use Ant chalk and you only kill a few workers and it only lasts a few days. You can get a commercial bait in Udon that will be taken back to the nest but it costs about 50 baht a go - Borax about 25 satang...

All this talk about borax reminds of Death Valley Days in the 50's and 60's.

I googled up 'seeking borax' in Thai on google: here's a lead:
ผงบอแร็กซ์ คือ น้ำประสานทอง เป็นสารอันตราย มีขายตามร้านเคมีภัณฑ์
ร้านขายยาจีน เรียก เม่งแซ

Roughly translates to: Borax is a hazardous substance, sold by chemical dealers and Chinese pharmacies. Ask for เม่งแซ (Maeng Saah)
You might try the Chinese pharmacy on Phosri Rd, across from the Bangkok Bank.

You could also try the store directly across from Ngeesoon on Phosri Rd. They sell a variety of chemicals....in the room alongside the one where they sell water filter supplies.

Great information.....