Always start your conversation by indicating your event date, time, and location. Professional references should include local banquet halls, photographers, caterers, and should not be paid for referrals. Remember, if you receive references from an entertainer, they're going to be only the best ones.
Different events require different attire.
It can be called a contract, entertainment agreement, booking form or an invoice for services. Whatever it's called, make certain the entertainment service has one and that everything you expect of them is in writing. If they refuse or are vague with answers, get up and leave. Having the commitments in writing will help them remember and protect your interests. Have them list all costs for your event.