Also, use the SCA as your source for merchants. The SCA holds their own internal events which always have merchant booths, so be sure to ask the SCA for names of merchants in your area. Though it will take more work, you can locate other crafts people willing to participate in your fair. Three options would be woodworkers, potters or leather crafters. An archery store for example, might be able to supply archers or a dance studio may have belly-dancers willing to practice their skills.
Food booths are good money-makers at Ren fairs.
Two easy period items which sell very well are turkey drumsticks and small pies made of meat or fruit. The pies can be baked ahead and kept on ice or served room temperature. Turkey drumsticks, always a favorite, can be cooked ahead or at the fair. For drinks, apple cider offered hot or cold is easy and inexpensive. Water wasn't a popular drink in the Middle Ages, but if you choose a warm summer day, bottled water should be available for sale.