Thanks to $programmer's recommendation of a good computer shop, I learned that the lithium battery used for the BIOS in my writing laptop, rather than being as I had thought out of date and hard to come by, is a design still in common use in hearing aids and hence I could buy one in the pharmacy for $4, which I would never have thought of on my own; it appears to be working fine, and I am much relieved, as well as determined to use the aforementioned good computer shop for any similar needs in the near future.
