I doubt you will have much luck tbh, but they might be the same size as something else? I have an old book from Quаltex that lists all the carbon brushes they ever made/make, I'll have a read later and post up what I find, then Stu might be able to lay his hands on them.
How worn down are they though? I've had less than 1cm before and it's been OK, I've got a turbopower 1 down to 0.5mm in the loft that runs fine, but it's only 400w so doesn't have the potential to wear down so fast.