Don't get a co-signer. I was a co-signer on an auto loan once and took a hit when the person missed a payment. It's just not fair to put someone else at risk because who knows what can happen?
Your best bet is to get a secured credit card from your local credit union and put down a sizeable deposit. Also, if you haven't done so already, see if you can rehab your student loans to eventually remove the default status. That's much better than having to wait seven years for the baddies to fall off your report.
If you haven't consolidated your loans yet, you can work out a rehabilitation plan with Direct Loans and they will remove the default status after you make 12 on-time payments.
I dont recommend a co signer or joint account. You would be better off rebuilding with 2-3 secured cards if your credit picture doesnt justify getting subprime unsecured credit for now
All my student loans are paid off (LATE but in full). I currently have zero debt but bad credit due to the federal student loans.