I'm not sure you can source a barrel at this point. I believe S&W used 952 (5") barrels for these when requested by the owners. Not sure if they did it for the compact model.
I was like you a couple years ago, when I was presented an opportunity to pick up a 3566 Limited which is the 5" version and it came with the 9mm auxiliary barrel. If it didn't, I would have passed on it due to same reasons you listed for ammo.
If the price is good, buy it, shoot the ammo, enjoy it, and then sell it!