I generally find it in the old hardware stores. Not every one has it , but it's a good place to start and the smallest I can find is fine. There is fine /med and coarse. Mine is made by Homax. I've never been on their site but it's on the package,
www.homaxproducts.com Hope this helps. If you can't find it let me know and I'll mail you a pad. A small amount goes pretty far.
Tom