I've got one I've been using since about '93, and it has a user-replaceable batteries. Mine has a round battery door on the back that can be opened using a quarter as a screwdriver. I just pop the new batteries in and clean/inspect/lube the O-ring. If your's is the same, note that the user's manual specifies using a coin, not a screwdriver. Oh, and wrt where to get the batteries, it uses two common button cells that're available at WalMart, sporting goods stores, battery stores, and possibly your LDS. I don't remember the #, but you can get the # off of the old ones.
When I bought mine I was told that older models had to be returned to Oceanic, though. They don't have the battery door on the back like mine. OR, I saw a web site one time where the guy gave instructions on cutting a hole in the bottom directly over the battery, replacing the battery, then filling the hole with epoxy (or something like that). Not sure it's something I'd prefer to do, though. And I think his instructions were for a different brand of computer.