Try these companies:

Phillips & Temro Technologies ( Also check out their " Cowl " brand spiral mufflers. )

https://www.hotstart.com ( Recently had one of my guys change the 6,000 Watt tank type coolant heater on the backup generator at work, Cat, 800 KW , V12 compounded quad turbo , 27 liter / 1650 CID, 1150 HP at 1,800 R. It uses 55 gallons an hour at full load , about 30 GPH at 50% . )

Kat's