I am seeking a source for corner flashings (interior and exterior corners) to go with the old cement-asbestos shingle siding. I am putting an addition on my 1920's home (which is already covered with this old siding) and wish to match the original. I have had no problem finding the shingles: GAF Purity 12"x24" shingles (non-asbestos but otherwise an exact match). However I cannot find anyone who carries or has a left over stock of the corner trim. I cannot even find any metal shops who will make it for me. Does anyone know of a source? I am in the Washington, DC area.