Need bedding help

Somewhat urgent but I’ve been medicating my boys with URIs for a while now and have been to the vet multiple times and the URIs won’t go away. I’m thinking it’s the ammonia in the newspaper bedding or something that is irritating their URIs and causing it to constantly reappear/not go away. I need to change bedding because I don’t want to medicate them for the rest of their lives. Problem is, I don’t know which bedding to use. Please give me some recommendations anything helps!!