I had almost given up on local beef until a chance encounter with an unreasonably tender sirloin steak at the Camp Cafe in Kanchanaburi gave us every reason to drive down to Kamphaeng Saen.
Kamphaeng Saen is home to the Kasetsart Agricultural University where they breed a sell KU Beef. You can buy frozen meat retail from their shop on the campus as well as in bulk. Although for some cuts you need to pre-order.
There is also an onsite Steak House for potential clients who might still be a little sceptical about the quality of what is the best beef I have found in Thailand.