...are there any flashcarts on the market that would work? Or does it have to be an older 3DS that hasn't been updated in a while? I've been reading up on some of the 3DS carts, seems like there's 3 big ones... R4i, Gateway and MT. Which one is currently the most stable/supported?
There are at least two flash carts that work with 3DS roms on the 3DS, however, the whole thing hinges on having a 3DS system with a firmware of 4.1 to 4.5 http://www.thegamerdepot.com/blog/which-one-is-best-gateway-3ds-r4i-deluxe-or-mt-card/ check out the link above to learn more details
If your N3DS firmware stayed on v4.1-4.5 and all you want is playing 3ds games, then you can buy gateway 3ds, after all ,this is the world's best 3ds game card and you can use it play nds games and gba games on your 3ds at the same time. But if your N3DS firmware higher than v4.5,and r4 card is your favorite, then you can buy r4i gold like this one : http://www.modchipfactory.com/wood-r4i-gold-3ds-flash-cart-for-3ds-xl-3ds-dsi-and-ds-lite-p-1.html , this is the best r4 card , 100% game compatibility and directly work on all nintendo console with all firmware version.