toucan, I'd try one of the hydroponic stores that have sprung up like mushrooms after a rain. I think it's used in hydro growing. Sea of Green has a store in Tempe, Tucson and north Phx.

I buy the BIG 2 cubic feet bags at the Home Depot on 90th St. and Mountain View in Scottsdale. I was there this past weekend (buying something unrelated but I remembered your post so I ran over and took these quick pics with my phone). They usually have 4-6 of these bags for $16.97. I imagine other Home Depots around the valley carry them too?

Let us know if you find a good source and all the details!
Happy gardening,
Grant

Here's the sign for pricing

Stack of bags.