You should be able to source one from a specialty battery shop. The swollen battery is a very bad sign and means it is a potential fire risk so correct disposal is urgent. As long as the specifications are still readable the battery shop should be able to source a replacement.
Just a note for others - the new Phonak remote - PilotOne II runs on an easily replaceable alkaline battery. I guess they decided that a rechargable battery was not really needed for this sort of device. I have had mine for 2 years and am still on the original battery.