Are there any boatbuilders close to you, give them a call, they often dump small pieces, you can scarph pieces together as well.. I recently scored about a cubic metre of off cut Corecell in various thicknesses from 12 to 50 mm from a well known super yacht builder which was all destined for the tip!
Consider using Gurit CoreCell M80 foam instead. It is a better core material and is easier to get. Noah's Marine online store in NY USA has 16mm (0.630in) x 24in c 48in sheets for $63. This is the TC version, which means triple cut, if memory serves. It has knife cuts part way through it to help it conform to curves. Give them a call if you have any questions.