You contact a entertainment agency and book someone or if you want a celebrity you have to get in touch with their management or PR company or representative and they will give you a quote and talk to your chosen celebrity and get back to you on whether the person is free and available/happy to do your engagement. Warning - celebrities do not come cheap some want hundreds or even a 5 figure sum to work on private functions.
Yeah definitely best to go through their agent. If you need to find contact details for a particular entertainers agent (address, phone etc) go to imdb.com, and subscribe to the 3day free trial for imdbPRO, that should get you the info you need.