I ordered a pin cask from G4 kegs a few years ago, I think back then it was close to $100 shipped. I imagine now it’s a little more expensive. If you can’t find any used, that’s a great option. They have firkins as well. You can also try uk brewing they’re in Lancaster PA I think? He sometimes has used pins and firkins in stock at a fair price as well - particularly if you don’t mind paying shipping costs.
I used to randomly scan Facebook marketplace too. I would alternate search terms between cask, keg, barrel etc and I’ve seen them on there before (DC area) so that’s an option too!