Do shops that sell used bikes normally also sell used parts?
They likely have "extra" bike parts around that are scavenged from bikes that were unrepariable, just be sure you know what your getting. There likely isn't any warranty included on used equipment, so if it was a "distressed" part that finally breaks a week after you've been using it, you be replacing it again. If you trust the bike shop and it is a reputable store that's been around for awhile, you should be fine. Nothing wrong with used parts, just so it fits and works correctly for its intended design, and doesn't appear to have been abused or been on a bike that was not maintained or has been wrecked.