i've ordered them online, and bought from a seal / bearing supplier.. and in a pinch, bought from a tractor mechanic.
dealer MAY have them in stock.. may not.
I doubt that an autozone will have them.. though if you got a number might order them. if they cross to a bearing in an old car you might be in luck.
same deal with napa.. you might be able to get them with a part number interchange..
it's an ez box to rebuild... i've done very many... i think all but 3 of mine have been rebuilt.. and 2 of those are PS.. which I try not to mess with..