want to take out a loan for a car can you help?

I want to buy a used car just to get around for school and work and stuff. decent maybe 2001. I need some advice on where I can take out a loan. I found this site for capital one loans but I realized it was only for franchised car dealerships and I might want to buy from a private dealer like on craigs list. I tried to apply for a loan at my local credit union but I wasnt approved because my monthly income wasnt enough. I had great credit too she said. Do you know of any sites online where I can easily fill out an application for a loan? And how does a co signer work cuz I might want to have my mom on there so I can have a better chance of getting approved.