If you used Online TurboTax in 2017, the prior-year PIN "might" be on a Federal Information Worksheet in your tax return PDF, but you'd have to get your PDF with all the worksheets to see, but there's no guarantee.
You don't need the prior-year PIN. You can sign your efile using the 2017 AGI, but that would require getting the 2017 return, too.
If you don't want to pay to unlock your 2017 return, you can try getting a free tax return transcript from the IRS. You
may be able to immediately download a 2017 return transcript at this
IRS website at the link below. If you don't qualify to open a free IRS
account there, you can order one to come by postal mail, which normally
takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript
