Ian,
The standard collets for the Boxford are the Crawford 3C. Need an adaptor to fit the 3MT spindle and only go up to 1/2 inch though that is enough for most model engineers. I would have thought every school Boxford collection had a least one set of collets. The usual suppliers are now doing them new in 1/16 steps probably because the flow of school lathes onto the market has left a mismatch in supply.
Of course it is noticable that the Americal hobbyists start of f with a 12" lathe and 5MT spindle so created the market for far eastern 5C collets and hence the 5C blocks.