[quote name='Genocidal']NES doesn't have flashcarts. There's too many mappers that would need to be supported that flash carts haven't been made. That doesn't mean, however, that someone hasn't found a game with the same mapper used as the Earthbound ROM and made their own flashcart like device.[/QUOTE]
You can easily put a rom into a NES cart, but you're right, the 'donor' cart needs to have the same mapper. But its frequently done, Earthbound Zero and the Final Fantasy 1+2 unreleased English prototype are the most commonly done. People sell these all the time, usually without trying to decieve anyone into thinking that its the real thing.