I just started fermenting 1/2 keg batches in a 20 gallon stainless kettle. Be forewarned, with a batch this big you will likely need to control the temp of fermentation. I keep mine in a plastic tub and toss in a little ice a few times a day to keep it in check.
People have used rubbermaid trash cans...search a bit...certain ones are food grade AFAIK. Another option is an HDPE 30 gal barrel.