I use to do this so have experience. Commercial agencies will charge as low as 7% (in England, where I worked) but this is usually for a job such as admin assistant or secretary. If I were you I would advertise the job myself through specialist websites and interview myself. Even if you use an agency you will probably have to do the final interview; after all, who knows your business better than you? I wouldnt trust an agency as they could employ someone you may think is awful.