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.
If you're hosting a big holiday bash for friends and family, consider hiring a clown to entertain the kids at a special "subparty." This way the kids will be out from underfoot, and they can enjoy the things they want to see and do while the grownups do boring adult things.