After seeing some of the things that Rock Auto has listed lately for the car I think they got ahold of Alldata or a GM Service manual and sourced every part no mater how obscure.
I was looking for some O rings when I changed the transmission fluid, they had all the internal parts to rebuild it listed, but no O rings for the drain and fill plugs. I liked their site much better when it just had things that someone who would work on their own car might need. I don't think there are many folks the want to DIY an automatic transmission rebuild.