It sounds very much like a Fiberboard Lap siding. Is yours textured or smooth? It is made in 4", 8" and 12" widths, 16' long, and is usually about 7/16" thick. It is also called "Bender Board", because a lot of the masonry guys use it to form curbs, sidewalks, etc. The 8" typically sells for around $11.00 and the 12" sels for around $14.00, depending on the market area. It is typically available from most any lumber yard, I know that Boise Cascade usually stocks it (in my area anyway). I stock it at my store.