70 Quart (17.5 Gallon) Igloo Xtreme Cooler. It appears to have a drain, so you should be able to convert this into a large mash tun.
As of this posting, Wal-Mart has this marked down to $41.20. Shipping to most US addresses is also free. Price, shipping and availability can change quickly. Check the product page for up to the minute price and availability.