Contact them and ask if they have a group that performs. People often know people who know people. Through the staff at the Renaissance Faire where we held our wedding, we hired a woman that played hammered dulcimer. Since the demand for Ren music is small, their prices tend to be low. You might try seeing if there's a local English Country Dance or Contra Dance community in your area.
Word of mouth is the best way to find a band or anything else for that matter.
Ask couples or parents who have recently had a wedding. Take an evening or two and go to all of the local clubs and bars. You get a ready-made audition this way. Call any local universities or colleges and ask around. Actually, the way I found the group for my wedding was by listening to the music at the Mall.