There are a number of websites dedicated to the sale and trade of salvageable vehicles, including eRepairables and Salvage Bid. The first site deals specifically with cars that need rebuild work, while the other trades in salvage from all categories, including theft, and the vehicles may have no damage at all.
It's also worthwhile to ask around local used car dealerships or check out some local police auctions for rebuildable salvage vehicles. These are good options for experienced vehicle rebuilders, and sellers may be open to negotiations over price. It's important for a buyer to make an accurate calculation as to the cost of the repairs needed.
When looking at a salvage car, remember that the price listed only gives an idea of the initial cost of obtaining a working vehicle. In some cases, the damage to the car might be superficial, but this doesn't mean the vehicle can be driven off the lot and used immediately. Most states require a vehicle that is listed as salvage go through an inspection; the cost of inspection varies by state. Insurance on a salvaged vehicle may cost more, and an insurer may only cover liability (accident) insurance but not be willing to cover breakdown repairs.