It all depends on which documents you need. Contact the IRS for tax related documents that include w2s, 1099s, and any income related documents. Other documents you may need to prove a dependent will be school and doctor records, lease(s), and anything else that will prove you had the person with you and supported them. The IRS phone number is 1-800-829-1040. The IRS generally doesn't issue refunds after a three year window, but does consider all years open when it comes to taxes owed. Good luck.
The IRS keeps your tax records for past years. You will need to contact them direct. Be prepared to put your phone on speaker and find something to read because, they will take forever to pick up. If you paid for more than half of the support then you have an argument.