I always suggest heading over to a local brewery and simply ask for a used keg. Odds are they will sell you an old keg and appreciate the fact you asked.
It also helps if you buy some of their beer, chat up a brewer and explain what you are doing.
The issue I've found is that by the time you retrofit the kettle the way you want you probably can buy a nice 15gal kettle from one of the LHB stores.
Adventures in home brewing, located in MI has converted kegs, with multiple welds available. Their kegs that have been secured "legally".