I replaced the brushes in my 15 year old 4" Makita Grinder last year when it was doing the same thing you are describing. The brush set was about $8.00. It works good as new. Took me all of about 10 minutes to install them.
$8 to fix a $60 grinder makes perfect sense.
Makita has parts / repair shops nationwide.
Here's a couple links you might find helpfull:
Makita - Authorized Factory Service Centers
Makita 9523NBH Parts List and Diagram : eReplacementParts.com