This is the brush I have used since I got the tip from another "No Poo-er"
This is a boar bristle brush and it's much healthier for the hair to use than a plastic one.
I've always hated to brush my hair, but the boar bristles makes it much smoother, it doesn't pull the hair like a plastic brush does.
I bought mine at http://www.ahlens.se that is a swedish store though, but I would think that it's easy to find a brush like this on several places. :)