We have the same type of roof on our rowhouse in Columbia Heights, and we’ve used Clerkin Roofing, (301) 888-1492. He generally has a selection of salvaged slate on hand. He had enough Vermont red slate to repair our roof with nearly identical slate. It was very reasonable, maybe $1500 to install several slate shingles and make a few other repairs. If that sounds expensive, keep in mind that the repairs were on the front side of the turret, 45 feet off the ground.