Talk to your homeowner's insurance agent first to see what deal they can provide you as a single day rider on your policy. Then compare prices with these online companies. Generally we have found that if you already have home owner's insurance (and it could be on one of your parent's policies), a rider on that policy is the best way to go.
This one costs about $125 to $150 for one day.
This one costs about $175 and covers extra things besides host liquor liability.
This one costs about $125 and covers other things too.
This one costs $175 and covers extra things in addition to host liquor liability. It also covers the day before and the day after your event for that price.