How can I obtain the latest available version of iMovie on High Sierra?

Unless you have another older computer from where you can download iMovie, or can find a download on the internet, you are unfortunately in a position where iMovie 10 is not available to you.

Beginning 2013 iMovie came installed for free on newly purchased Macs. Since your computer is a 2011 Mac the iMovie installation did not come with it. Unlike the Photos app, iMovie is not part of the operating system software so it would not be included when you upgraded to High Sierra.

Since you do not have, nor ever have had, iMovie on your computer there is no way to get an iMovie download for High Sierra. And, you need Mojave 10.14.6 to download the current version of iMovie.

Possibly you can find an iMovie 10 download on the internet. You could try a search. Perhaps Amazon?

You can get iMovie 6, however, at the Mac Repository website:

https://www.macintoshrepository.org/24547-imovie-6-hd-6-5-1

iMovie 6 is an old version with completely different architecture than the new iMovie versions. Still, it is a great program, very user friendly, that has many effects and features that the newer versions do not. It doesn't have Green Screen or Picture in Picture.

-- Rich

You are welcome. :)

Yes, there are other video editing apps out there. You can check in at the app store website and type in "video editing apps" and there will be several listed, includin Filmora. Final Cut Pro is also listed, but costs $299. Whatever one you get, check its specs to make sure it will work on High Sierra.

-- Rich