Where is Final Cut Pro X software be available?

https://apps.apple.com/us/app/final-cut-pro/id424389933?mt=12

This is product you want. The X was removed from the name with version 10.5, as it's been removed from the operating system name.

Just to add that the application itself was always called "Final Cut Pro".

The "X" was added with version 10.0, as a way to distinguish it from the legacy Final Cut Pro (versions 7 and earlier), and also because at the time Apple used "X" in multiple places - starting with the name of the operating system (Mac OS X in 2000 with version 10.0, then just OS X with Lion 10.7).

The X appeared in the App Store and marketing materials, and the application became widely called "Final Cut Pro X", but in your Applications folder that "X" never existed. If you looked at the menu bar it said "Final Cut Pro".

Since Sierra, the "X" was dropped, and the operating system became known as "macOS", so it became clear that the "X" was going away. Finally, this year we got Big Sur, which is macOS 11. If not for so many years trapped under the "X", we'd be in macOS 18 or something...

Final Cut Pro is available in the Mac App Store.

The current version is 10.5.1, and requires at least Catalina 10.15.6 or Big Sur 11.0.1.

What is your version of macOS?