Shotski: You don't buy spores, you buy mycelium. Mycelium is the stringy underground part of the mushroom. I got mine from "Skyttegårdens Østershatte" a danish company producing oyster mushrooms. I paid something like 15 dollars for half a pound. Rather expensive, but it's possible to keep it going for quite some time, taking a bit of mycelium from one fully outgrown bag and use it to inoculate a new bag.
In the states, you could buy the mycelium from Fungi Perfecti, that is Paul Stamets company. He's quite the expert when it comes to mushrooms.