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You know the dealer is probably not going to fool with selling brushes for a window motor. Repair work is so expensive by the hour, it doens't make sense sometimes to pay him to do it, according to the bean counters.
I would try your local hardware store, the ones that have an assortment of brushes in the back room. And like the other poster said, you may have to get something close, and file them down to fit. I have done this also on other types of motors, and it worked.