Do you mean that you might need more than the school designated cost of attendance, or that you might need more than your stafford limit? If it's the latter, it's easy because you can borrow GradPlus loans to cover any difference up to the cost of attendance. If it's the former, it gets a little tricky. First, you need to try to work with your school to see if they'll increase your cost of attendance. Lots of people at my school have been successful at this for various personal reasons. If that works, then you can use GradPlus loans to cover the difference. If that doesn't work, then you have to look into loans that have worse terms and that are heavily credit based. Bank of America has a loan for this situation, and I would guess other lenders do as well.
I agree with Doctor Bagel, if you can get the school to increase your aid that would help. If not there are plenty of private lenders out there who would be willing to lend you money based on your credit score.