I'm looking into replacing at least one of the window motors on my GQ since the windings are shot (blackened in at least 1/2 of it). I tried just cleaning up the brushes but it didn't make an difference. It did get me thinking about replacement brushes though and I think the best option (if you can't track down originals) would be to shape some from some Tamiya/Mabuchi or similar motor brushes. I've got some spare brushes from a high power motor that could easily be filed down to the right shape since they're a very soft metal. It'd be fiddly but would work. Other than that, I'd go for some 2nd hand motors from any other Nissan. I'm looking at a set of Skyline motors on Ebay and swapping over the parts I need.
I know this is an old thread but as I was browsing for a solution to this problem, here's my contribution. I just bought the cheapest carbon brush that I can find from our local hardware, and took some measurements from the working one. (i didn't take notes, as I was scratching the makita carbon brush on the fly) I then cut it using dremel, after a few minutes... there you go one issue that I experience when I put it back on the vehicle is that it wont go up, but i will go down, I removed it from the window, grounded it to the vehicle and let it run for a few minutes, I think that did set the new carbon brush in, after that rolling the window up is a breeze.