Proper model railway locomotive wheels come in all sorts of different sizes and styles but can be expensive. However if you want something that rolls well they are your best bet. In the US Walthers seems to carry everything, in the UK there seems to be a large number of smaller outfits. Your best bet for finding a retailer is probably to buy a model railway magazine and look at the adverts.
Dapol have some of the old Airfix and Kitmaster moulds of various locomotives and wagons and the kits include moulded plastic wheels and I think proper model railway wheels for the wagon kits. They are OO scale which means they might be a bit small and as the moulds are old there is a lot of flash but they are cheap and some of the larger locomotive driving wheels might be just what you need.
Minicraft have a kit if a steam locomotive and carriages that is worth a look, I think there is another thread here about one.
Regauging plastic wheel sets from OO or HO to S should be easy enough as you can sleeve the axle with some plastic tubing.
Belfast Caboose used to import American model railway items including some S gauge stuff so I have had a chance to look at it at the local shows but the rolling stock was eye wateringly expensive. The track seems reasonably priced though. When he retired some English outfit took over as distributor.