Places to obtain a car radio code for free include the car manufacturer, a local dealership and the car's previous owner. Car manufacturers and local dealerships can generally provide you a car radio code if you can supply them with your vehicle identification number and the radio's serial number.
If you purchased the car from a private seller, he may still have the car radio code and be willing to provide it to you; if from a dealer, contact customer service, and ask about the code.
You can also check the paperwork that came with the car. The previous owner may have written the car radio code there. Also, check the glove compartment, as some vehicles have stickers in these areas where the car radio code is written.