The model 66s don't have bobbin cases. They are built in, just drop the bobbin in and push on the little 'ejector' button to get it out. The slide plate can be bought from any parts dealer, probably Billy (Lostn51), or any number of listings on Ebay. The slide plate goes on easily from right to left, (under the pressure foot!)
As others have said, it's not hard (or expensive) to find a treadle cabinet to put it in.
Enjoy your treasure!