What do they want to use it for? Is cast iron required? This is a guess, but I would imagine a large
ceramic vessel would in most historical periods be much easier to create and cheaper than a similarly sized cast iron one. Big cauldrons strike me as usually used for the sort of thing you use a crock-pot for anyway -- long slow cooking.
Here is a 2-gallon ceramic crock for $36.99, although this does not come with a lid.