Make friends with someone who owns odd pets. Tarantulas use the stuff too. When it gets poopy and has to be discarded they can give it to you.
If you are impatient, buy a "coir brick" from Petco. You add water and it grows like mad. It is about five or six bucks but you get a lot of light, lovely soil.
Oh, and look below for my Coffee Grounds post. Those guys have a LOT of the stuff, and it is good (mixed with leaves or torn newspaper it is a good soil substitute.)