You should have gotten one with your kit. If not, talk to Fran.
I'm not sure why there is a rush to buy an adapter- it is a part of the kit, so if you don't have one and need it because you want to run an aftermarket wheel (like the one in the kit), call Fran to get one.
But the one I got was from
Sweet Mfg- Quick Release Assemblies & Components
So, no, it's not a bespoke part for the SLC. Over the years, the adapter may have changed as to suppliers, etc. But this one looks like the one I have.
One piece that probably is custom is an adapter that is a machined aluminum piece that adapts the normal adapter to a specific wheel. You may not need this if the steering wheel hole pattern matches that of the column adapter. If it doesn't one of these billet-style pieces will be needed in addition to the provided column adapter.
If you are using a late C6 or XLR steering wheel, no adapter is needed.
Alternatively, there is a wide array of adapter manufacturers and feature sets, so if you wanted a collapsible one (though the column itself is collapsible) the Pfadt one looks good. And one from NRG allows the wheel to flip UP to increase leg room when entering the car.
So there are choices, and options. But I imagine most people will use the standard adapter that comes with the kit.