Where would I locate a my 2018 ".tax" file?

[CassLO]

If you used the Desktop program, the .tax files and PDF files are usually in My Documents in the Turbo Tax folder.  Or try searching your hard drive for all files ending in .tax or .tax2018 or the Recycle bin. In the Desktop program you have to make sure to manually save it. In Windows 10 they might be in the One-Drive area. Check your Recycle Bin and any other places you use or backup to like Drop Box or One Drive or any cloud backup services.

"VolvoGirl Level 15"

Please also see the TurboTax FAQ below:

Find your tax data file (.tax file) in Windows