where do you find the original installer for the mohave 10.14 for mac

Apple only provides a single, full Mojave 10.14.6 installer which is an aggregate of all Mojave updates.

You can check your MacPro hardware compatibility and a link to that full Mojave installer in the Mac App Store and update this from the following site using Safari only:

How to get old versions of macOS - Apple Support

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS Mohave App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS Mojave.app in Applications, empty trash & restart, then a new copy of a more recent version can be downloaded..

I get this error message, no matter what I do. I am trying to upgrade from High Sierra 10.13.6

Okay, you are so awesome. I didd the empty trash and restart. I am downloading it now. Thank you so much for your help.

Great to hear my friend, take care!:)