Start local when searching for used auto body parts. Body panels are expensive to ship and could get damaged in transit. If looking online, search for sellers who provide high-resolution photos of the parts. You want to see as much of the part as possible to make sure it doesn't have any damage. Additionally, pay for online purchases with a service that offers buyer protection in case there is something wrong with the part.
Many cities have self service salvage yards. If your car is fairly common, chances are a yard has it, and you can closely inspect the needed parts for damage. Don't forget to bring your own tools. Browse your local classifieds for cars with major engine damage or listed as "for parts only." The seller may be willing to sell you the part(s) you need. Alternatively, if you have the room for a whole extra car, you have a ready source of parts; you can also sell the parts you don't need and possibly make money on the transaction.
If you can't find your car locally and your car type has an active online forum, start your search in its classified section. Many car forums have some sort of vetting system, so you can be sure you're dealing with an honest seller. It is also easier to get extra pictures of the part.
Online auction sites have large selections of parts, but they require extra caution. Make sure the part you're looking at is an exact match for your car, and use part numbers if possible. Get as many pictures as possible, and save them to your computer after the auction ends so you have evidence if the part that shows up is different than what you ordered.