Sadly, clowns are as likely as any other entertainer to be seduced away by a higher-paying gig. That being the case, insist that your clown sign a contract (and not in disappearing ink, either). That way, they have to show up -- or face the consequences.
When looking for a clown to hire, it might be easiest to deal with a talent agency or an event planner. They can usually offer a number of acts for comparison, and help you choose the one that's right for you. This process can save you a lot of time and money.