Hi @hellomacos
Search online for ''PR-062334 battery'' to find suppliers that suit you best. (see part number on original battery at 57 seconds and 1:40 minutes on replacement battery into this [https://www.youtube.com/watch?v=3whLR9dvUqg|video])
Couldn't find a waterproof backdoor for the Hero 1 only [https://www.ebay.com/itm/323555997357|this] which is probably not what you want.