I went to Davidson's Restaurant Equipment to look for a 15 gal. heavy duty stock pot to convert into a brew kettle and found and aluminum one that is on for $82.49 from $153. The quality looked good to me, and the price was excellent. They also have the 20 gal. version on for $95.79, I am not sure on the regular price, but, it would obviously be more than $153.
Check them out if you are in the market.