o that blank spot on your belt is calling out for a knife? Tired of eating with your hands? There are several options to obtaining a vaguely period knife. Keep in mind that most faires enforce a peace-tie policy.
Standard knife shops like Cutlery World are generally useless for anything but sharpening blades. Sometimes you can find local artists who make knives and who may be willing to do custom work; but custom work isn't cheap. Gun shops sometimes carry some knives.
I purchased my belt knife at a gun and knife show in San Francisco. Many gun shows have vendors who also sell knives. There are also shows devoted solely to knives. Ask around at local gun shops or knife shops for shows in your area. Gun and knife magazines also often list touring gun/knife shows.
Atlanta Cutlery World - 1-800-883-0300
Museum Replicas Limited - 1-800-883-8838
Make your own! It's fun and simple. Use an old file, and after you have the shape right, heat with a torch until a bright purple (yes, purple) and toss in cold water. This is about the proper temper. (File temper is way too brittle for use, and the tip will chip right off...)
Carve a handle, or find a nice piece of bone. There are undoubtedly books in your public library. I wouldn't want to do knifemaking as a trade, but as a one-time effort it's fun, and if you take your time, your effort can be very rewarding.