I actually just dealt with this last week. Try calling local breweries, if you have any, or try calling local dive bars. I lucked out by finding one on Craigslist. If you frequent a bar and can get to know a manager, they may sell you one for the price of their deposit.
Worst case scenario, have a party with as many friends as you can and you pay the deposit and split the rest of the cost for a keg and drink it dry!