Where is the boost gauge sending unit on a 1992 Bonneville SSE w supercharger?

The 1992 Buick Bonneville SSE with the supercharger doesn't have a boost gauge as standard equipment, nor does it have a dedicated boost gauge sending unit. The factory system didn't monitor or display boost pressure. Any boost gauge you see would be aftermarket and would need its own sensor installed in the intake system. You'd have to decide where to place that sensor yourself, likely tapping into a port or using a T-fitting in a suitable location on the intake manifold or charge pipe.