Just grab the git-clean-stale-local file and put it somewhere in your PATH (e.g. /usr/local/bin), then make sure it's executable (chmod +x path-to-the-file).
It should then appear within your usual Git completion (so you could type git clean-stale-local) and work just fine.
This assumes a Bash-compatible system (Git Bash on Windows, for instance, and any shell on Linux/OSX).