Gee, um , what brand piston is it? there are a few places to search.. any local shop can buy rings from Pro-x or wiseco.. or you can go online to a Pro-x dealer and look through their catalog and pick out the part number and order it from them..
you can find loads just by google -ing it.. but what piston is it?
Ive used them, but... sometimes I got screwed over.. they didnt send the actual part, but what they thought was the "equivalent" part.. great if theres no mistaking the part!