The bearings are a tapered roller type and not found very may places. The Filling Station has them for 1/2 to 1 ton trucks. If the lower bearing race is worn on the shaft, you will need a new shaft since the race is machined into it. The race in the lower cap is also part of the cap and is non serviceable.
When I needed a new shaft for my gear box, I purchased a 1 ton piece and cut the shaft down to 1/2 ton length, the box and all the internals are the same 1/2 thru 1 ton, not sure of bigger trucks. Parts manual list 41-46 all up to 1.5 ton as the same bearing.
http://chevy.oldcarmanualproject.com/parts/1929_54/401.htm
