To get current version of iMovie 10 you would need to upgrade to Mojave 10.14.6. There is no reliable link to an earlier version of iMovie 10. However, if you have or ever have had iMovie 10 on your computer, but perhaps deleted it, you can get a download of the most current iMovie 10 version that your operating system will run. Here's the procedure:
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
-- Rich