I don't know if this is against the rules, but you must buy devices that hook up to your computer. Then you plug the cart into this device. A program on your computer will then read the data off the cart and viola you have a rom.
Alternately, there is quite a number of development tools out there which can be used to author your own software for the systems which intail generating ROMs that can be used in an emulator or a flash card.
