I am trying to find one that would work without modifying the firmware or anything, kind of like the sky3ds but for DS games only. Is there a firmware requirement at all?
All that work on newest firmwares on O3DS work also on N3DS, N3DS does not have any unique anti flashcart measures compared to normal original 3DS models; I can personally say DSTWO still trucks.
These are older (I think). I would stick with the R4i Gold (cheap and reliable) or for near guaranteed support in all future firmwares, the Supercard DSTWO.
If it's r4isdhc.com then it's a good, cheap card. Only get it if it's a recent year. Actually I know the only difference between 2015/2016 is the sticker but they don't want you to know that.
These are older (I think). I would stick with the R4i Gold (cheap and reliable) or for near guaranteed support in all future firmwares, the Supercard DSTWO.
Not really needed since Nintendo doesn't update their whitelist anymore. And dstwo sucks battery. But I would get the R4i Gold if it's cheaper than what's mentioned.