I would recommend going to your local, high quality English schools, and checking for who is available.
Most teachers at these schools tutor on their spare time, and I would say they are the best qualified.
Although you may be able to find competitive prices from someone who is not teaching, and only tutoring...
I would say, quality-wise your best bet is to get at least a reference from one of the well know English schools in your area.
Or at least the schools will be able to give you some high quality references.