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.

there exist flash carts where you can insert SD cards and load them on your gameboy

at least the guy who wrote https://news.ycombinator.com/item?id=26857743 has custom cartridges for his games

There are tons of cartridge solutions available. Just search typical sites like tindie. That said, the most highly recommended solution is going to be the everdrive.