Co signers for a loan?

after all the federal loans and grants, i still need to come up with $11,000 for my first year of college. My tutition planner said that if my mom was denied for the parent plus loan i'd get another $4,000. that still leaves me with $7,000. Do i now have to turn to private loans? i know i need a co signer since im only 17, but if my mothers credit isnt the greatest, who else could co sign for me?