Brushes don't work well on chins, you want a comb. Which comb depends on what you are wanting to do, are you preparing a chin for a show or just wanting to make a pet one look nicer during shedding? If it's just a pet chin a fine toothed comb works well, the one I have looks like this, same style but different brand. Some people even use flea combs.
If you are looking to groom the chin for a show though, you will need a professional comb set. They are basically a comb made of needles, they get in to separate each hair. I'm not sure where to buy those, and they can easily hurt your chin if you don't know what you are doing.
Also in case you don't know, unlike most animals, when groom a chinchilla you start from the tail and comb towards the head. Also give a bath after combing so loose fur the comb missed falls out in the dust.