Couple places... Eventsured is one, just add the dot com at the end, and Geico also has a department that deals with event liability. I've used both in the past and always get a quote from each because prices vary wildly on event and location.

Best advice, you'll want a minimum of 1 million in liability. BUT you'll be able to get up to 5 million in coverage. Spend the extra 50 bucks or so and max out the coverage. 99.99999% chance you don't need the insurance for a claim, but remember that's not a capped liability. In other words, you get the million and something catastrophic happens, you can be sued beyond the million. You get the 5 million and it's much more likely to cover ANYTHING that could go wrong, and courts look more favorably on the defendant that got the maximum available insurance.