How can I get a loan without a co-signer?

I need a $15,000 to $20,000 loan for a car that I am looking at buying from a private dealer. I have a good credit score of 790 but do not have enough credit history to get approved for a loan without a co-signer. I have tried multiple banks and haven't had any luck. I have also looked into military banks due to being in the Army Reserves but most of what i find requires you to be active duty. Any help or personal experiences of things that have worked would be greatly appreciated.