Marcos is the only place I know of that sells new ones.  You can sometimes pick them up on ebay but I have not seen any for ages.  There are a few other tricks you can try - look at coil stops on slings and see if they have the thicker type that can be swapped over with the flippers if they are in better shape.  Also check the outhole kicker on single ball games, this often has a very nice stop.  You can also knock the slug out of the frame and fit one of the screw-in types you can buy, but make sure you the correct length as some are too long.

I have scavenged enough to keep my Sterns going for many years, but I was at one stage looking to 'convert' to using Bally coil stops from the same era.  As far as I can see, this would just involve putting new holes in the plate, the trouble is the holes overlap, so it might get a bit messy.

cheers
Ian