That’s sort of the catch-22 of taking stuff apart and repairing things. You’ve got to do enough of it, and break some things, in order to figure out how to get these types of things apart without breaking them.
I don’t carry replacement batteries, or sell anything of the sort, but I’ve seen lots of LiPoly packs in various shapes and sizes for sale on ebay. I’ve had a lot of success ordering parts like that.