Where can I find the tax extension filed previously through turbotax?

The IRS Form 4868, Application for Automatic Extension of Time to File U.S. Individual Income Tax Return

should be included in the 2018 PDF download from the online tax return.

To access your current or 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 bottom of the screen and on the section Your tax returns & documents click on Show.  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.