Many of the festivals around here sell temporary vendor permits. Usually those are provided by the city or county. You have to meet some basic requirements (three sinks, warming aparatus, aka, chafers, instant read thermometers, etc. ) but it is not as strengent as say, what they expect of a diner kitchen. You pay as part of your booth fees, and an inspector "approves" your area prior to prep/serving.
Andy /Smoke on Wheels Competition BBQ - www.smokeonwheels.com