I see these Mauser sporting rifles for sale with the claw bases but no rings. Is it possible to find replacements?

In short yes it is but it aint easy !They are all individually fitted to match the front and rear bases.
Sources for new rings would be Recknagel or EAW.
Secondhand a good source is Egun.de but you need to brush up on your German ! They appear on Ebay also.
And if you do buy a job lot get a good set of files,start filing to fit your new rings and when you have a small pile of filings buy another joblot of rings ! I went down this road.tiresome but fulfilling.