Either flashable cartridges (so you need to also buy a flasher), or more expensive cartridges with an embedded µSD card reader. Just be careful to buy cartridge

Either flashable cartridges (so you need to also buy a flasher), or more expensive cartridges with an embedded µSD card reader. Just be careful to buy cartridges with enough storage for your game(s). I personally own an "El Cheapo" from Australian maker BennVenn, but they only make small batches a few times a year.