Lee Valley also sells the cones and I have the pair. I've only needed to sharpen one so far and I just rotated the fine cone back and forth between my thumb and finger for a minute, rather than in a drill, and the chisel cut thru wood like butter. I would think they might go bigger than 1/2" which is my largest but I'm not home to go check them.
There is an auger bit file that is for filing that type bit. LV also sells them but someone like Axminster should also sell them but you could go to LV's website to see what it looks like and get an idea of price.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.