Up until the start of the great ammo drought, I thought I had this one well in hand. For me the "goes (almost) anywhere battery" is something that's legal pretty much anywhere that guns are (in the US - rest of the world, you can pretty much forget about having a gun w/o a LOT of paperwork at best). It would be something that you can draw supply on pretty much anywhere. It would be useful for self defense, and secondarily for hunting.
For me the ultimate "goes (almost) anywhere" battery was a GP100 in .357 mag backed up by a stainless Mini 14 and 10 bangers. Legal pretty much everywhere, but now that NY and a host of other locales are restricting both magazines and guns, I replaced the mini with a Marlin 1894C, also chambered in .357 mag. My EDC .38 snub would "work" too (and is carried everywhere it is legal to do so) but is too limiting as my only sidearm. What I lost on range with the mini I made up for with logistic compatibility with the Marlin. In the early days of the ammo drought I could still pretty much count on finding something in .38spl or .357mag albeit at inflated prices. But those days have ended at least locally.
Gone are the days when I can enter a Wally World and restock on .38's or .357's at will. But I can still get 12 ga shotshells pretty much on demand. So I replaced the Marlin with a Stevens 350 (Ithaca 37 clone) and slugs w/ some 00 buckshot on hand.
While its not totally unsatisfactory I would like to go back to the days when the mini graced my goes (almost) anywhere bag. If the ammo shortage ends and I can assure myself of not accidentally "felonizing" myself by carrying an evil 10 banger across some arbitrary line on a map, I will add the mini back. Though I will probably keep the Stevens in the bag too.
So, anyone else out there maintaining a "goes (almost) anywhere" bag? If so, what's in yours?