Darryl:
Any hard foam insulation will do.
The Owens Corning web site does include ProPink, http://www.owenscorning.com/around/insulation/products/propinkrigid.asp
but ony the thinner stuff. But that’s OK..do the math and use 2 layers, each 3/4″ thick. The idea is simply to use ANY insulation you can find locally, then be sure to seal any other air spaces with spray foam.
Always be sure you do not go overboard by making the inside smaller than 19″ since 18″ is the approx. diameter of a beer keg.
Cheers!
Steve