Andrea, I usually make my own from three components. I like coconut husk chunks (CHC) because I don't have to water as often, and it doesn't break down as quickly as fir bark. That's important here in the summer because it's blazing hot and very dry. The medium you choose really kind of depends on your growing conditions and your own watering habits. If you live in a warm humid area CHC may not be the best choice for you. That said I mix 4 parts CHC (soaked a minimum of 24 hrs) with 1 part perlite, and 1 part charcoal. That seems to work for the majority of my plants. I know others mix in Sphagnum moss, Hydroton, marble chips etc. Like I said you just keep playing with the recipie until you find one that works for you.
I have to order the CHC online. Here are a couple of places I have purchased from in the past and been happy.