I grow Oat Grass for Prego and Polly. I get my seeds from www.poopsiecat.com
. They absolutely love it! From what I read, cats will crave greens. They will normally get a small amount from the belly of their prey, but domestic cats don't have prey.
Go out and buy two small terra cotta pots, a bag of soil and some seeds. It takes about 5 minutes to plant them, and the grass will last about a week before it starts to turn brown. Plant one pot and when the grass grows, put it down for your cat and plant the other. This way you can rotate the pots and always have grass. It's much cheaper to buy pots, seeds and soil than it is to buy those pre-planted things, and it's really easy.
Your cat will love it!