I'm not a Chevy guy but I don't believe 72 SS Chevelles came with 3 speed manual transmissions. I could be wrong I suppose. The one thing I do know is that they came with the Muncie 4 speeds or TH-350's and TH-400's depending on the engine options.
As to where you might find one new, I don't have a clue. Hope your search is successful though.