Similar to Civil War re-enactors, this group specializes in the Middle Ages. It's a non-profit educational organization, started in the 1960s and has chapters all around the U.S. Members research and recreate many aspects of the Middle Ages as a hobby. They are frequently asked to do demonstrations for schools and other youth groups. If they are able to participate, it's appropriate to offer them a small stipend for their efforts. Don't worry, they won't use dangerous weapons.
Kids especially love to see the knights in action.
But, since they will probably work for free and show up in wonderful Medieval costume, accept any offer of services from SCA members. 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.