If you need to hire classical musicians, consult someone who is immersed in the local professional classical music world. An excellent place to start is your local symphony orchestra. Don’t call the conductor or the concert master, call the marketing director and tell him or her what you are looking for.
The person that holds that job will know every classical musician affiliated with the orchestra, and may well be aware of one or more musicians that work in quartets or small ensembles for private events. Virtually every orchestra of note belongs to the national symphony organization found at www.symphony.org.
The world of professional classical musicians is a relatively small one, even in very large cities, up to, and including New York. A lot of professional musicians have worked together and known each other by reputation or professional experience. In a similar way, the people who are involved in the arts specifically the musical arts tend to know something about classical musicians who are working in group ensembles for events.
Most classical musicians who make a living in the music world belong to the American Federation of Musicians. You can consult the union local for suggestions how you go about screening the musicians that are suggested by a hiring hall is an issue you may need to avoid, however.
Large hotel and country club catering departments put together formal events on a weekly basis. Catering directors at such facilities are going to know of classical musicians that are available for a variety of events. The trick here will be eliciting cooperation from an employee of a facility that you may not be using for your event.
Most marketing and catering directors understand the value of good public relations however, and know that helping out a non-client can lead to recommendations down the line.
In many cities, the fine arts performing facilities are operated by a group of city or county employees. The programming personnel for these facilities will be familiar with many of the classical musicians in the area and the groups with which they play. A large number of classical musicians have multiple jobs in their repertoire.
There are only a few cities where the symphony orchestra is strong enough to pay its musicians well enough so that the orchestra is a full time job. Many musicians supplement income by teaching and/or playing for small ensembles that work on the commercial circuit: weddings, formal parties and so forth.
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Consulting the music department at your local college might be a productive avenue of inquiry as well. More specifically, consulting faculty members who are also working musicians of which there are many.
Classical musicians tend to be supportive of one another on these sorts of inquiries. If the person you reach is not working for the type of ensemble you’re looking for, you’ll likely get a recommendation for another musician or resource.