Internet. Since it's too rare for me to go to any garage sale (and due to the fact that I live in a deserted place, screw it!), it's my last resort.
But if you're lucky enough to stop by an EasyCash shop (in France), you may find an old console (I got my GameCube from them with a Zelda Collector Disk, so, there can be good deals sometimes), or old games. I even remember having seen a Zelda II cartridge for the NES once!