As a house warming gift, my mum got us an Bunnings Homelite electric lawn mower in 2009. After eight years it conked out. I opened it up and found the carbon brushes were very worn down. Turns out it's impossible to get any kind of spare parts for these Bunnings sold mowers.
Carbon brushes in general are a pain, I've only ever bought one tool that had replacement brushes, surprisingly a cheap Chinese Dremel clone. Rule of thumb for these things, if there's sparking it's the brushes, if there's intermittent operation and/or decreased power and concussive therapy (shaking/slapping it) helps it's the brushes.
As for replacing them, search eBay for your brush based on dimensions, it's usually a couple of dollars for a set of ten or so, and there's literally thousands of them available from various sellers.