I think you mean the lever to adjust flow. It is part of the faucet. The adapter is just 2 small pieces of metal to connect the faucet to a liquid quick disconnect.
I was interested in this for my first kegging setup, but went with a cheap picnic tap because of the comments about the Perlick faucets on an earlier thread:
Several places. The QD adapter for the Perlick flow control faucets (and Intertap) was only available at one place that I know of; CHI Company. Everything else is easy to find.
I still use my Marks 2 Keg Cleaner to pump starsan through the taps every couple of months. This is especially needed after hoppy beers. The flow control taps can get bits of hops stuck in them. When this happens, the beer is very foamy when served.
I don’t know for sure. I believe that they have helped me through one or two times when I have overcarbed, but, I have never served a beer intentionally above 2.5-ish volumes CO2.