If you are looking for Homasote, forget about looking at the big box stores like Home Depot and Lowe's (not sure if Menard's has the stuff since it has been a while since I have driven over to the closest Menard's).
You're best bet for Homasote, if available at all in your part of the country, is one of the smaller but well-stocked locally owned lumber yards, if you are lucky enough to have one in your area. It is generally used as an insulating material in colder climates, but has been largely replaced by foam products of various types.
