Where?

I’m not going to post the link but they are called Twharf D Cell Batteries 1.5V/5000mAh D Batteries Micro.

I really could not believe it when I charged them for the first time. Here is a pic https://i.imgur.com/2LoyZlR.jpg

Those look incredibly useful. I'll be ordering some in the future if they are still around and don't seem to have any reliability/safety issues (I'm slightly wary of Lithium batteries being crammed into places that haven't been thoroughly tested for that application, given the tendency of these things to catch fire. A Mag-lite with lithium batteries would be great but resembles a pipe-bomb a little too much for comfort until they're proven.)

I don’t know about their reliability, but the sheer novelty of recharging them is something else.

I’ve only done it once in six months for the cat feeder and it hadn’t run out.

No fires yet. Presumably the danger would be in the wiring and power electronics. I’d guess the battery is off the shelf, though I haven’t torn one down—they aren’t exactly cheap.

My favorite part is the action on opening the end. It is slightly spring loaded, but also limited, it won’t come apart. Someone clever designed it.

They have other standard sizes as well, but I haven’t tried them.