I had this problem recently and wasn't able to track one down. I resorted to buying a new crossmember with a mount from a 280zx which are readily available.
My understanding is that the GM mount would only work with a new crossmember unless you did some modifications yourself. The one I saw referenced is on Amazon. You can search "Energy Suspension 3.1108R Transmission Mount for GM " but need someone else to say if it would work with some modifications or not.
There are several new crossmembers available that work with more readily available mounts such as Zcardepot, techno toy, and Troy Ermish.