Good morning, Curt;
Any of the plastics houses should have a polyurethane foam sheeting that comes in varying thicknesses up to about 4 inches. I preferred the "closed cell" polyurethane. If there is not an industrial plastics supply house where you are, try looking also for a furniture upholstery shop.
My memories of the Doskocil people come from the cases they made for firearms. They worked great for other long thin things that I did not want to see bent or damaged.