try fish gravel or sand
 

I am having the worst time trying to find growrocks, I was told to try fish gravel or sand, is there any other more readily available medium I could use instead and still get optimum results?

If you use gravel use the porous rocks you can find because they hold more oxygen than smooth stones. Some members use perlite because it's cheap but I haven't tried it yet. You also have to keep it from clogging the system because it's small by using paint strainers in the bottles.
Ron