I suggest two carts R4i Gold and Ak2i. Both of them I've used and download plays seems to work nicely on them.
That one is a DSTT clone and I haven't tested it to really say if it's any good. I always suggest spending the extra dollar or two to get a better quality product.do you know if this one is a good one too? http://www.nds-card.com/ProShow.asp?ProID=146 cuz its a little cheaper and seems to have wifi and some other plus stuff. It will work with the download play feature too?