How to replace a cracked boiler plate?

I have a 850000 BTU Natco Firecoil hot water gas boiler that I use in my laundromat in NJ. Its side plate made of cast iron cracked when the boiler was not even heated and it flooded the basement. There are 5 or 6 bolts holding this plate. What kind of gasket may be required and where to get that. Any ideas how to replace the cracked plate will be highly appreciated.