Well First try some one like TEPCO 01584811575 they deal in lucas electrical parts.
You only need brushes that are bigger in size than the ones you need.Then file em to size.
I usualy sort any early ones like this .
cheers
mick
Agree with mtb.........just find some suitable (or larger) brushes and modify. (they are essentially just carbon). Was defeated recently though by a VERY expensive vacuum / shampooer (Ok, maybe can't spell tonight). Absolutely impossible to get to the brushes. It seems if you can afford their product in the first place, you simply have to buy a complete new motor. Would name it, but can't remember!