If your costume can wear Renaissance-, medieval-, or Scottish-style clothing, it can easily make a big hit at these events. Make sure you run through the place out of suit, then return in suit, to see what to expect. Most if not all Fair grounds are outdoors, with some being without shade, some being with shade.
If you're lucky, you may be able to go there at no charge on invitation by the organizer as an actor. You will need a changing area either inside the grounds or outside. Usually a tent with some water and fans will do well. If you have none, you can use your car for doing so, but there's not much privacy when you have to retreat there to behead and cool off.