What you want is a motherboard that would match the HP Z800 "revision 1" workstation. That might be a "revision 3" motherboard, for example.
Use google to search specifically for "HP Z800 spare parts" without the quotes included.... the top result is what you want to see.
Almost at the top you'll see 3 motherboards.... the latest one is the one you want (highest number). There have been posts in this forum on the actual Assembly P/N and Spares P/N (different numbers, for the same motherboard). You can troll eBay and sometimes figure these things out, too.
Later is better, and the motherboard revision does not equal the workstation revision. It is confusing, but can be done.