While I don't know where you can find copies of old games to play on your Mac, I can tell you that Lode Runner is available on XBox Arcade as well as on the iPhone. It's an updated version of the game, though, and not the original stick figure graphics.
I'd love to be able to play Karateka, Choplifter, Skyfox, Track and Field, and other classics on my Mac. Anyone remember the dungeon crawler Wizardry? I loved that game. It came on something like five 5.25" floppies.