That’s not a Worpdress plugin. It’s custom JavaScript using the Google Maps API:
https://boys-brigade.org.uk/wp-content/themes/Boysbrigade/company-finder/js/app.js
The Google Maps API is pretty fun and easy to work with. I’m sure in this economy you could easily find a developer willing to help you build something similar! Check out codeable.io and jobs.wordpress.net
Best of luck!