The SQLSaturday Dallas Team used this company for buying Event Liability Insurance.
http://www.privateeventinsurance.com/index.jspThis costed about $150.00 for an event with under 500 attendees with no liquor served onsite.
We also bought a seperate insurance from the same company for the after-event party where we served liqour. The after event party was a bar which already had a liqour license. The after event party insurance also costs $150.00
This just an example and you should check on the ability of the company to actually pay up if something goes wrong.