Built on the same E-350, E-450 chassis as C motorhomes, by commercial divisions of some of the same companies (Thor, Forest River) you get the same range of sizes and performance, walls, ceilings and floors less well insulated, more windows (usually not opening), none of the house stuff, less room below floor (usually no basement space).
I suspect you won't find them on the market until they are pretty worn out, if actually shuttles. A few moderate mileage example hit the market here from failed small town mass-transit efforts, though most of those will have a lot of space used up by wheelchair lifts.
If I were looking for something C-sized to do a conversion, I think I'd rather find a E-350 or Express 3500 school bus. Those are still steel body, and usually become available at lower mileage, although often much older model years.