While replacement batteries are easily found on Amazon and AliExpress, capacity tests by community members such as Makho figured out their true capacities are so low that sometimes they last less than an old, deteriorated, original battery. With the increased usage of battery draining cartridges such as flashcards, the situation is even more dire.
However, one can use an old Nintendo battery and a new modern lithium ion pack to substantially increase the battery duration by making a custom one. In fact, this one advertised itself as a 900mAh battery and actually delivers 920mAh - making it substantially better than the real Nintendo provided batteries from back in the day, lasting surprisingly long.
If you guys want a tutorial for how to make your own, do let me know!
Yeah, I repaired a few batteries of different devices that way. Cut it open, keep the electronics and connector, replace the cell.
The issue with replacement batteries for old devices is, that they are often equally as old as the device. They've just sat on a shelf for that duration instead of being in a device.