In this video my buddy Inbred Jed sharpens my ms661 with his triangle file in hopes that sharing his knowledge will help you get your saws razor sharp!
I bought this triangle fine on the phone when I called Madsens in Washington State. +1 (360) 736-1336. They also have a website but it’s terrible. I don’t know the brand of the file or where to buy it online so give them a call and order one!