I just bought the thunder group 40qt on eBay. Its 4mm and very sturdy. 3 days from order to my door.
I seasoned it today on my SQ14 burner. 7 gallons to start, 40 minutes to reach a boil(started with verrrrry cold water from the hose, 35 degrees outside). Boiled for 60 minutes, 5.5 gallons at the end of boil. Not too shabby, I'm very happy with my purchase.