Getting financed for a car?

How can I get financed for a car if I am an independent contractor, and have poor credit, without having a co-signer? I have tried Carmax for a car that was about $20,000 and only one company would finance me if i was'nt an independent contractor and if i had a $4000 down payment. They suggested to look at a car that was less expensive, but I am not sure if that is even going work. Can anyone give me suggestions?