I was never into flash carts despite having one for backing up my carts to emulate for when I sold/broke my system. Seeing as you posted this in the original NDS sub-forum, I'm going to answer for that and not the 3DS. Get the original one! Cheap cheap cheap! Also, use the WoodR4 firmware.
I'm talking about the phat NDS and DSLite. If you use a DSi, not sure what to tell ya (Acekard maybe? Never had a DSi). Can't point you to where I bought mine though. The site I got mine from, I think 7 years ago, isn't operating anymore. :/