I use Ultra-tec so I can't help a lot with Graves products but if
you go to a good hardware or motor repair shop and buy a set of
brushes that are close to the ones you need and a bit oversize you
can file them down to fit your motor and they will work as long as
the spring is about the same, assuming yours has the brush and
spring attached? If yours is the carbon brush with no spring and
just drops into the slot it would be no problem making one from a
larger new one.
There is some difference in brushes though, some are solid carbon
and some are a mixture of metallics in carbon, usually copper and/or
silver. But if yours is the solid carbon I think one can be made if
you can't find the right ones. And even the others might work OK I'm
not completely sure on that.
There are a few very good faceting lists using yahoo that might be
of some help.
Sorry I can't be of more help.