Does the factory sell parts?

Does the factory sell parts (do they have a catelog), or do I go to the authorized dealer in my area? Being a member of Trailblazers, do I get a discount and if yes, how to I get it? I would like to upgrade my water heater from a gas unit to a gas/elec unit, anyone make that change? Can that change be made on a 2001 3023 TM. As usual, thanks to everyone who took the time to reply,