Depends on whether your tractor was equipped with a Ross or Gemmer steering gear.
If Ross, it takes 20 5/16" steel bearing balls.
If Gemmer, it takes 207802 roller assembly (Timken 5BC), 207565 lower bearing cup (Timken 6A) and 207570 upper bearing cup (Timken 7A). One of those cups is extremely expensive whereas the roller assembly is cheap. Timken products are available through your auto parts store. Timken will try to sell you a race to go with the bearing, decline this since the race is made integral with the worm.