I think everyone goes to dealership or ZZP. I don't think it's that much though. Stuff like that is usually only a couple bucks. The dealer might not have it because they don't sell parts for the superchargers, just the entire assembly. That means you can't buy things like the coupler from the dealership and you have to buy the entire supercharger!
There are parts dealers for the eaton supercharger though, such as ZZP, w body store, and other places on the internet.
I'm just gonna do a quick search for the plug on here to see if there is a GM part#.