I keep thinking I should go to a feed store or "co-op" and buy some bulk cover crop grain. Or pay 5-10 times as much at a health food store, but at least they won't expect me to buy 50 pounds.
Paying Baker Creek $4 for one ounce of hull-less oats might have been crazy, but it was the cheapest seed I've bought from them ever.
I think that oats, rye, barley and hard red wheat can all be used as cat grass. I have plenty of red wheat berries from Fred Meyer. I have some barley from a supermarket, but have not yet tried to sprout it.
My cat keeps rejecting cat grass, but I keep trying different grains. He'll chew on grass or even weeds outdoors, but if I give him something, he turns up his nose in disdain.