Where can you find a blank DS cartridge

You cannot get a blank DS cartridge. There are manafactured for Nintendo by a company named Macronix. They use Mask ROMs, which means the game is programmed into the cartridge during the manafacturing of the cartridge, so it can't be overwritten.

They are numerous flash carts for the GBA and DS that can often hold a memory card. ROM files are put onto the memory card and can be played back on the DS.
Popular ones are R4 and Acekard.

http://en.wikipedia.org/wiki/Nintendo_DS_storage_devices