My dad was so proud of how much money he saved, when he bought a $300 four-function electronic calculator, because a mechanical device would have cost $500.
It did get some practical use, though. My mom was a second-grade teacher, and if her students were very good, they got to check their answers on the calculator, which really impressed them.
I was just using my 15C last night. If it ever gives up the ghost, I’ll definitely want to replace it.
The weird thing is, I bought it in the '90s, and I can’t ever remember replacing the batteries.
Ah, yes, the Space Cadet Keyboard.
http://catb.org/jargon/html/S/space-cadet-keyboard.html
I have a 15C app on my iPhone.
I know that’s available, but I prefer the tactile feedback of the real thing, Plus, I often look things up my phone while I’m doing math, and it’s easier to have a phone and calculator than to switch between two apps.
I have the HP-25 app on my phone. Sadly my real HP-25 won’t charge anymore, and since it uses ni-cad batteries, things are a bit hard. I use the app simply because, after all these years, my fingers know exactly where to go. And as implemented on the iPhone, it has a haptic click when you push the buttons 
Still not quite the same as the real thing.
I have a few other HP calculators, but once you go past the basics, going to a more powerful computational environment is much faster. There are a huge number to choose from. So for a calculator, HP-25 it is.
[I know you know all this] As far as I know, the HP-15C has not been produced since 2011 (that was a “limited edition” version brought back by popular demand, so, who knows, could always happen again) and used ones sell for multiples of their original price.
However, the ROM image is available, so there are various emulators available— including virtual calculators published by HP [?] on their web site and for your phone, as well various clones. The nice thing about those “Voyager” calculators is, they are reasonably small. Smaller and lighter than an iPhone 12, and the original consumed only 0.25 mW…
Better than nothing, but I hope my original will keep going for a few more decades.
Now that I think about it, I don’t remember what batteries it takes; are they still made? I’ve got an old 35mm camera with a light meter that clips on the top, and I couldn’t find the right battery for it. A smaller one with the same voltage seems to work.
LR1154 button cells? (e.g., Panasonic LR44 or Duracell MS76— the owner’s manual lists a bunch of recommended brands)
those you can get
I just checked my owner’s manual and it says A76 (alkaline) or 357 (silver-oxide). Those are both still available.
Wow, bunch of old fogeys, here… I’m from the HP48 era, and that’s what I emulate on my phone.
Although, my backup calculator is a slide rule, so…
It needed to be done (~ 5:00):
I have a collection of early calculators, many of which used ni-cad batteries. I’ve had decent luck going to a local battery store (such as BatteriesPlus+) and talk to the technicians there. They’ve always been kind of excited to rebuild a battery pack, which they said is a nice change of pace for them instead of just selling plain, standard AA and car batteries.
So I noticed a Logitech keyboard with “F” keys: F1, F2, F3, etc. But it also has “G” keys: G1, G2, …, G6. Not to mention “M” keys (memory?):
Macro key-set selector. The “G” keys can be loaded with stored keystroke sequences. The M<number> buttons select among three different set of “G” key defintions.
The HP12c is still the best financial calculator out there. I got my first one over 30 years ago and still use one regularly. HP hasn’t really changed its functionality at all over that time and it still is a best seller.
Like most electronics, it’s more than half the cost today than a new one cost 30 years ago.
My 41CX that I’ve had for over 30 years died today 
Wait, I thought that HP had gotten entirely out of the calculator business a decade or more ago, and wasn’t making any any more.
So sad! During college I spent hours programming my HP-41C in 1980 (the original version). I actually had an interview upon graduation with HP in Corvallis where they looked closely at my notebook with about a dozen programs. I wish I knew where it went. I’ve collected about a dozen programmable calculators from that era; they all had a slightly different programming style, but HP had the best setup with the 41.
I don’t know the difference between HP, HP Inc., etc., but if you go to the official web site and click through a couple of times you get redirected to “official licensees” (Royal Consumer Information Products and/or Moravia Consulting) which are selling the calculators. (As well as “collaborating in the production, distribution, marketing, and support of HP branded calculators” and development of new HP calculator products.)