You will be talking about the main screen as the outer one doesn't have one.
It's definitely not advised to apply another protector over one installed.
Rather best to leave the original one for now and use that until you feel it needs replacing.
There will be third party retailers that will do protectors. Personally I'd probably go with a Whitestone Dome one if I were to replace mine.
Or go to a Samsung approved agent or Experience Store to get it properly replaced and have it a warranty covered.