There is lots of info out there, they have their own forums. I am putting together a bowling ball mortar but I am leaning towards pneumatic instead of powder. With powder you only get 4 shots per pound but it’s easier to build. With pneumatic I can shoot all I want for the cost of gas for my air compressor, can use the rig as a sawdust cannon and a bead seater too.
For a bowling ball most people use a high pressure Oxygen or various inert gas tanks. The tank you are looking for is (approximately) 9.28” in diameter with a circumference of 29 3/16” , .350” wall thickness and a bowling ball slides in will a little room to spare.