(I've been working in Property & Casualty insurance for almost 20 years.)
You should be able to buy commercial insurance as a sole proprietor. Some insurance carriers may have specific requirements, but it's very common to have your situation. If you only want coverage for the festival, you could look into short-term commercial liability insurance. (This was once the "hot new thing" in small business insurance; it hasn't really panned out that way but it's still generally available.)
I would recommend talking to an independent insurance agent local to you, because insurance is regulated primarily by individual states. Independent agents represent many different carriers, so they are likely to have a solution for more unusual situations. (State Farm, Farmers, Allstate and American Family are captive agents, so they only have one carrier they work with.) You can also go direct with online quotes yourself, but you're likely to get better advice working with someone else.
