I just bought some ArtCare mat board and machine cut overmats from Lodima. I think they represent the best value out there in pre-cut mats, especially in regard to the overmats. They know how to cut the openings to allow floating the entire image in the window with extra room at the bottom for my signature, and they know the correct offset from the bottom of the mat for the window in any size. These things are very important to me and it takes a photographer who exhibits to understand them and deliver just what I need.
All I have to do is tell Michael what size picture on what size board, whether portrait or landscape, and I'll have just what I need in a day or two. Highly recommended.