When looking for a singer to hire, don't overlook the Internet. There are numerous specialty sites that will allow you to locate singers in your local area -- or, if your budget allows it, singers from outside your area. They all include the relevant contact information.
Looking for some rousing singers of an unconventional sort? Look into hiring a barbershop quartet. It's a fun and traditional version of the vocal group that most people have heard of but few have directly experienced, and is ideal for nostalgia events.