you can always wait till some original game cartridges fall from the sky (hopefully on a soft surface), then you're legally allowed to use a rom of those games
or maybe some cousin was going to throw away some 100 game cartridges and gave them to you instead... ^^

once you're in possess of these prerequisites, you can download a rom of those games from one of the many sites specialized on that (just google).
If I'm not wrong, you can download roms even without possessing the game, but should delete them after an evaluation period of 24 hours.. (not sure though)