Using Google, this farm was the only supplier I could find in the rural areas of northern Kanchanaburi province.
Very nice product, with a decent choice considering how deep into the National Parks you are. The English speaking owner was pleased to help, he saved me a long drive back into Kanchanaburi City.
You will find the farm on the left hand side just after you pass "Nature Club Resort" at about 11km before the Mon Bridge. Speak to the shop owner in the outdoor shop and what you need will appear.