How can a user select to only see the services in the area they want?

If a user goes to my directory, and then clicks on a particular category of it, for instance Link removed at members request how can they then choose to only see the companies in the area they live in on that page?

For instance, on that physiotherapy page, if they only wanted to see what was available on that page to people in Essex, how could I make that possible for them?

Thanks

Sally