Do you have a unit that is not working? Does that unit save any settings(which would require a battery)? Since its way out of warrantee, just pop the lid and have a look. Odds are if it has one, it will likely be obvious(probably a CR2032-style) round, silver and about the size of a quarter. Odds are also pretty good that it will be soldered in place, and while you can find that style on line, you can also solder in 2 pieces of solid wire, make a loop(for more contact area), hold the loops against each side of the battery and wrap with a high-tack tape like this:
https://www.uline.com/Product/Detail...B&gclsrc=aw.ds
Home Depot has small rolls that are not expensive. I've done 3 devices this way and all are still in service after several years