I filed my 2018 federal taxes with the free system. I now need a copy of my federal return. How do I get a copy of my 2018 federal return?

With the Turbo Tax Free Edition or the Free File website? 

Try printing from the Print Center

Log back in your TurboTax account and "add a state" to open the return back up (you are not really adding a state, just to open it back up), then

Click on Tax Tools on the left side of the online program screen. 

Then click on Print Center. 

Then click on Print, save or preview this year's return.  Choose the option Include government and TurboTax worksheets

How to save the PDF

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-pdf/00/26234

If you cant find it,  That's common to end up with multiple accounts.  First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address.  Run the tool against any email addresses you may have used     

https://myturbotax.intuit.com/account-recovery/