Part Numbers:

Piston: 8642079

Housing: 8673039


from another forum "yes you can get one from the GM dealer but you will probably pay dearly for it. They are available aftermarket very reasonably.

GM Price is $65 list for just the piston, you will also need seals, etc.

I have a couple of used ones and new seals here if you can't find one locally.

Jake"