where do i obtain Mac OS Mojave software for my MacBook

Excellent posting from @a body and or How to get an older version of macOS Just be sure to open the links in Next page Only in Safari as FF / Chrome will not work Also, make a Time Machine Backup Before advancing further.

https://discussions.apple.com/docs/DOC-13571

Delete it and try again using Only Safari and a quick, reliable and stable Internet connection preferably over Ethernet connection