I need a rechargeable MN21 battery. I was dumbstruck finding out those are nowhere to be found. They do exist in some solar powered LED garden lamps and candles, but not available as a battery pack.
After a bit of searching, it looks like there simply isn't any such standard. The ones you've seen were probably custom made for whatever reason.
What do you need it for? According to Wikipedia, it's really close in size to a 10280 lithium-ion battery. If you have enough room and can do some soldering, you could use a boost converter to step up the voltage from one of those to 12V.
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