Wild Bird Seed, Rice, Rye, Oats, Coffee grounds, etc, etc. You can grow on anything really if you add some nutrient solution and sterilize it.
PF Tek is simplest though, and using rice for it is the best. The people who have been working with the PF-tek have tried dozens of substrates and there is a reason why the PF-Tek is recommended for beginners. It's easy to acquire(buy rice and grind it into a flour), and it's nutrient rich.
I will always support the pf tek, for small scale, or a simple method of spawning to bulk.
Honestly go to the store and buy a 2 pound bag of rice for like 3 dollars. Grind it up and wablam.