I called my brother this morning. His Yanmar came with Bridgestones. He tried Chinese replacements from 2 manufacturers which are about 2/3 the price of Bridgestones, but they wore out fast. He's back to running the Bridgestones.
6-14, 7-14, and 6.00-14 are common sizes on the front of 4x4 tractors. Some farm co-ops stock them. Not that actual tire size varies across brands labeled as the same size, be sure to measure before putting out the cash.