This is really obscure, but really easy.

It turns out that the full iMovie 9 app is included with the freely available iMovie 9.0.9 "update"... it just takes a little know-how and elbow grease to liberate it.

Here's what you do:

Download iMovie 9.0.9 Update
- https://support.apple.com/kb/DL1574
Double click iMovie9.0.9Update.dmg to mount it
- open ~/Downloads/iMovie9.0.9Update.dmg
Use Terminal.app to expand (not execute) the pkg archive
- pkgutil --expand /Volumes/iMovie\ 9.0.9/iMovie9.0.9Update.pkg ~/Downloads/iMovie/
Rename Payload to Payload.zip
- mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip
Unpack Payload.zip (to Payload 2)
- open ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip
Move iMovie to the Applications folder
- mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload\ 2/Applications/iMovie.app /Applications/iMovie\ 9.0.9.app

Tada!

Enjoy!

Command line Instructions (for Terminal.app)

You have to do some of these steps from the command line, so you might as well do all of them from the command line to make it faster and less error prone:

# 1. Download the iMovie 9.0.9 update package
curl 'https://updates.cdn-apple.com/2019/cert/041-98154-20191017-1fbef983-8d87-49af-83b1-edecc007ce2e/iMovie9.0.9Update.dmg' -o ~/Downloads/iMovie9.0.9Update.dmg

# 2. Mount the dmg
open ~/Downloads/iMovie9.0.9Update.dmg

# 3. Expand (unarchive) the pkg
pkgutil --expand /Volumes/iMovie\ 9.0.9/iMovie9.0.9Update.pkg ~/Downloads/iMovie/

# 4. Rename `Payload` to `Payload.zip`
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 5. Unpack `Payload.zip` (it will get named `Payload 2`)
open ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload.zip

# 6. Move `iMovie.app` to the `Applications` folder
mv ~/Downloads/iMovie/iMovie9.0.9Update.pkg/Payload\ 2/Applications/iMovie.app /Applications/iMovie\ 9.0.9.app

# Enjoy!
open /Applications/iMovie\ 9.0.9.app

First Run is SLOOOW!

Make sure you unplug any cameras or phones from your computer before opening iMovie for the first time, otherwise it'll take a while to catalog everything before you can begin to use it.

Compatibility

This should work, in 2020, on all of the MacOS releases:

10.12 Sierra
10.13 High Sierra
10.14 Mojave
10.15 Catalina

If you have an older version of OS X, you may just need to download an older version of iMovie 9:

All iMovie Versons

As long as the file size is about 1 GB, it probably contains the full version, meaning that a slight modification of the process outlined above will likely work.

Get iMovie 10 via iMovie 9

If you try to get iMovie 10 from the App Store and you don't have the absolute latest version of macOS you might get a warning like this:

Would could not complete your purchase

iMovie can't be installed on "Macintosh HD" because macOS version 10.14.6 or later is required.

HOWEVER, if you first install iMovie 9 with the method shown above, when you next visit the App Store you will find that you now have the "Upgrade" option.

Neat!