Congratulations! It takes a LOT of use to wear-out a set of brushes.

Many electric motors of the same size regardless of brand or application use the same carbon brushes. Start by downloading a manual online. Look to see if it contains a parts list then contact the mfg to order. If they are not offered measure the brush or photograph it and search online or contact a power tool service center - they will stock many brushes and can probably match up a set.