I was looking around the other day for a 55 gal drum to make a rocket oven with and could not find one that was in good shape. I did see a few old waterheaters with the outside metal in good shape. Could one use the metal shell of a water heater as an substitute for a 55 gal drum? I know from watch the Rocket Ovens DVD how the drum is used. Also I do not know if the metal from the water heater would take the heat or fail within a few years.
You should never forget that every creature has its purpose in the cycle of nature and can also be very important to humans. Sepp Holzer's Permaculture
Many places to find a used 55 gallon drum: Oil change places, online classifieds, metal scrap yards etc. Usually with a bit of looking and a bit of patience, one can be found locally so don't give up.
The metal in a water heater tank is much thicker and therefore much heavier than a drum. I would say it would last longer because of this though. It is also smaller in diameter. So if either of these characteristics are not what you were looking for, then a drum is the way to go.