Where can I find the Mojave installer? App store says it is not available.

First make sure your Mac will support Mojave. Basically you must have a 2012 or later model but there are exceptions for some MacBooks:

macOS Mojave - Technical Specifications

If your computer is supported, then:

https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

Once an OS version is obsoleted, you can usually find its "hidden" App Store download link by going to Official Apple Support and searching using the name of the OS followed by "download." Example; mojave download

I'm having the same issue. I have a Macbook Pro 2015 with High Sierra 10.13.6 and want to upgrade to Mohave in order to use FaceTime group chats. Catalina isn't an option because my older 32-bit Adobe apps (like InDesign) will not run on it.

Same problem and neither suggestion works. The link pushes to itunes and then it says it's unavailable. I've got a MacBook Pro 2015.