You'll have to either wait to see what flashcarts are supported with the installer is made, or go ahead and buy one and hope it works. Most folks are assuming it'll be the AK2i, but there might be others. It won't be useful on the 3DS latest firmware, so you couldn't use it as is right now, but still might come in handy.
http://nds-card.com/ProShow.asp?ProID=160
If you want a DS flash cart that is supported on the latest firmware, I recommend the Supercard DS2+, but only for DS games. Do not use it for 3DS games for any reason. You will most likely brick your system if you tried.
http://nds-card.com/ProShow.asp?ProID=135
Right now, though, we don't know if you will be able to install the custom installer to any flash cart without 3DS homebrew, or using a second flash cart on a DS.