Any wood-based cart updated after 2013 (or at least advertised working on 3DS FW 7.0) should do fine.Now that nintendo has apparently given up trying to block carts, and cfw unblocks them again, i'd liek some updated info.
Here's what i'm looking for in a ds flash cart
1) 100% compatibility. this means every clean rom works. EVERY. NO EXCEPTIONS. Since there are no new ds games
2) Same with dsi enhanced games. EVERYTHING MUST WORK
3) no time bomb. very important.
4) all homebrew works as well. (auto-DLDI patching)
5) as cheap as possible.
6) SDHC support
7) CFW 3ds compatible without a separate CIA. if it does'nt work in a dsi or ow 3ds that's fine.
8) Reliable. doesn't randomly freeze, lose saves, etc.
Haven't tried Ace3ds+ but seems alright. I recommend personally: http://nds-card.com/ProShow.asp?ProID=507. It'll run anything you throw at it. And NO "timebomb". This is www.r4isdhc.com.cn and Wood-based carts like this don't have it.