There are online shopping sites that sell used car parts. Examples are eBay.com (which is a fairly popular site) and Car-Part.com. But with Car-Part.com, you have to specify the car's model/make, year, part you're looking for and the area you want to buy; then the site will look for shops nearest you that has the available part you're looking for. To be honest, it is much better if you buy the car part in person so you can examine them properly.
To guide you on how to buy auto parts without getting screwed, you should read this site.