If you need to buy one, put a WTB here in BF or check at Ben's Cycle and at Tree Fort. If they still existed I'd say Niagara and A. E. Bike. Another thing would be to head to an old shop and see if they have any used ones in a "rainy day" parts stash.

I don't believe it's required to have a Campy sleeve, but I've never tried to make one.