If your cat likes cat grass, you can buy pre-made containers that have dirt and seed, you just add water. Often these packages will instruct you to add water, cover with saran wrap and place in a warm area for a few days. When the grass starts to sprout, uncover and let it grow. After it is about 4 or 5 inches, you can let your cat take a nibble.
After your cat is done, tuck the grass away and let it have some more time to grow. Offer the grass periodically to your cat but take it away and intermittently allow it to grow.
You can also do this yourself by buying cat grass seed (which is often packets of wheat seeds).
For more information on why cats like to eat grass, please read
Grass Roots - Why Cats Nibble Grass.
