I take them out. I measure them. I call this place.
https://www.carbonbrush.co.uk/
He makes up my order (occasional he has them in stock).
He mails them to me.
I will be doing this tomorrow for the 9" grinder that dung the bed on sunday cutting a few beams up.
What belt sander is it Mike? I've got a Makita 9404 that eats through brushes within the span of a few months but it does get very heavily used, as I said above I've had non-genuines that have been eaten far quicker and possibly cost me a new armature since it needed replacing afterward.