I am filing taxes under a new username this year and need to access my 2015 tax documents. I don't want to pay again, but it seems that's the only way I can get them

[Alma · Accepted solution]

To access your prior-year return, you will need to make certain you are logged into the same account you did your taxes in for that specific year. It is possible to have more than one user ID linked to the same email address. It is also possible that you registered your TurboTax account for the prior-year with a different email address so make sure and check any that could've been used. If you used TurboTax CD/download software last year, then your return will be saved on your computer's hard drive wherever you chose to save it to and not online in your account. If you used TurboTax Free Edition last year, you will need to upgrade to Plus or pay upfront for the version you are using this year in the same account before you can access prior-year information from your Tax Timeline. If you used a paid version of TurboTax Online last year then you should still have access to download last year's return from your Tax Timeline. If you are just needing your 2015 AGI, there is no need to upgrade to Plus, you can get that from your Tax Timeline in the account you filed from last year.

https://ttlc.intuit.com/replies/5574744

You also always have an option to get your prior-year transcripts for free or pay for a copy of your return from the IRS: 
https://www.irs.gov/uac/how-to-get-a-copy-of-your-prior-year-tax-information