Local classifieds, both print and online, are your best option for finding used muscle cars. Thoroughly inspecting the car is vital whether you're looking at a restoration candidate or a ready-to-drive vehicle.
Unless you have a very high budget, the car you purchase is likely to need some work. Carefully consider how much work you are willing and able to do or to pay for someone else to do before starting your search. If you live in an area where the roads are salted in the winter, or near an ocean where there is a lot of salt in the air, you may have a harder time finding cars without rust damage.
Local classifieds are the most efficient way to find cars that you can inspect in person. Online classified websites can get a lot of traffic, so be ready to move if a good deal pops up.
Automotive forums are a great place to find cars. Many users post "build diaries" detailing the work they put into their cars. These logs give more insight into the car's history than a simple advertisement. Forums usually have some sort of member vetting system so you can be sure you're buying from an honest seller.
If all else fails, drive along country roads looking for cars with "For Sale" signs. If you notice a car sitting behind a house mechanic's shop for several weeks, ask him it he is willing to sell it. Put the word out to your friends and family that you're looking for a muscle car.