How do I purchase Plus to access my 2017 return when I did not file in 2018 and it is past the deadline to access online 2018 filing? I need it ASAP.

You will not be able to upgrade to the Plus edition for tax year 2018 since the TurboTax online editions for 2018 have closed to make way for the 2019 online editions.  Once the 2019 online editions become available in December, at that time you can start a 2019 return and upgrade to Plus.

You can also get free transcripts of your federal return from the IRS - http://www.irs.gov/Individuals/Get-Transcript

For a fee of $50 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf

Or contact TurboTax support during business hours and see if there are any other options available for accessing a prior year online tax return.

Support is open from 5am to 5pm Pacific time, Monday thru Friday

Use this website to contact TurboTax support during business hours - https://support.turbotax.intuit.com/contact/
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax