Where Can I Find Insulation I Need to Build a Passive Solar Heat Box?

http://www.motherearthnews.com/diy/solar-heat-box-insulation.aspx

I ordered the Heat Grabber Plan from MOTHER EARTH NEWS. The plans were originally published in the 1970s and the product listed for insulation, Thermax, does not seem to be generally carried by big box stores in my area. Do you know where I can get this insulation?

Thermax brand insulation is made by Dow. To find a distributor in your area call Dow Customer Information Group at 866-583-2583.

— Heidi Hunt, assistant editor