I would think you would have an easier time finding a whole expansion bit instead of just finding the movable cutter. They aren’t terribly expensive in my area of the country but I do know that they can be hard to find in other locales. In order to match parts to your bit we would need to know the brand and size at a minimum. Ebay has a lot of the bits if you can’t find one in your area. I wish I could be more help on the subject.