Michael,
Looking at the white wheel in the image above I can't say whether they would fit or not. But around a month ago I was browsing a local salvage yard and picked up a 5" x 1/2" x 5/16" bore 'Dean Abrasive Products' white stone with the idea to manipulate it onto my old GMF dental lathe.
They had a 'heap' of them on shelf which they acquired from a liquidation auction. They were marked at $5 each, covered in dust as was the shelf they were sitting on so they had not been moving many. I offered $2 and they were happy to take it confirming they had been sitting there for years.
Quite possibly with 5/16" arbour they're probably too small for your machine, but if you 'can' use a dozen or so, let me know and I will go and check them out again as the yard is only a couple of K's away.
Cheers
Kim