How can I get a needed tax return for Fafsa if I am on Social Security disability?

Get a tax form (1040, 1040A, or 1040EZ, whichever works for you) and fill it out with the information you have, even though you don't need to file it. Then you will have the necessary line numbers and amounts to use in your FAFSA application. If that means all zeroes, then it will be zeroes.