Chinese supermarkets. They have enormous stainless steel pans and they also have the drop in inserts for catering warmers.
I got an enormous stainless steel hood from a fryer for a dyebath once. Now I have a huge sink that I keep beside the green house for washing out plant trays, etc. It needs a 1000mm kitchen unit to fit it into a utility room. Came from Ikea for £12 as a seconds....Typha has the dubious distinction of going shopping with me and I actually did buy the kitchen sink
...no idea how you'd support it to boil water in it though
How about an old jerry can ? Cut the top off it ?, or would a steel gas bottle work likewise ?
M