A medium is a person with the ability to communicate with spirits. Many professional mediums hold séances for clients.
The American Association of Psychics is a good resource for learning more about mediums and locating a medium near you. However, it’s important to note that this is the most expensive option for communicating with spirits. Mediums don’t work for free, and they can charge a lot of money, even when they don't successfully communicate with your ghost. This is recommended as a last resort, unless you’re rich.