Cottage Ceiling Boards

[Doyboy]

I am wondering if anyone knows where I can buy some replacement ceiling boards for my cottage. They are a 1/2 inch thick, wood fiber material that come in 4ft by 8ft sections. I believe that they are/or are similar to a product called "Homasote". I see them in a lot of old cottages up north. I had a roof leak last summer and need to replace about three of them but I can't find them anywhere. Thanks for your help.