Why do you need the 2018 AGI? It cannot be used e-file a 2019 tax return since a 2019 return can only be printed and mailed so there is no need for a 2018 AGI.
To access your prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the section Your tax returns & documents. Click on the Year and Click on Download/print return (PDF)
If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created. TurboTax does not store online any returns completed using the desktop editions.
Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript
For a fee of $43 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf
