Booking magicians and other entertainers has become easier due to more and more resources that are available. Even the phone book has numerous entertainers listed that are available to hire locally.
When booking an entertainer you may be able to book them from a website that they run or have provided on a business card, the websites have even been seen listed in advertisements in the yellow pages. If a website is not available they generally have either a phone number of an agent or their personal business number in which one could call to book the magician or entertainer.
Depending on the popularity of the entertainer one may have to book them far in advance. However, if the particular entertainer that you want has already been booked there is typically a plethora of different entertainers to choose from. In all actuality it is fairly easy to book a magician or any other entertainer.