Right now, many of the backyard chicken keepers are experimenting with growing fodder to feed chickens. I've been doing it myself for awhile. See: this post on my chicken blog for all the details. If you're already growing fodder or you happen to plant wheat fields, then you already have the seed. Go ahead and throw a handful in a pot, mist daily and let the cats enjoy. (I use dirt in mine, but we'll get back to that in a second) If you don't happen to have any wheat, there are plenty of places that you can buy wheat seed online or in feed stores in much smaller quantities. I've found a 1lb package to be around $3 online. I've also noticed that prices aren't that different between regular and organic varieties. Score! Eventually it's going to need replaced and it'll be cheaper to just buy the 1lb package then to buy a single pot.
Ok, lets get back to the pet store. As I mentioned there was nothing else in the pot. Seeds are created with enough nutrients to sprout and start to grow. They have no problem getting to be a few inches tall with just water and a little bit of sunshine. At some point they do run out though. This is where dirt and compost (or compost tea) come in handy. This is also where a tip proof pot comes in handy, darn cats! Watered and fed like any other house plant it will continue to grow for months. I trim mine down from time to time and throw the pieces to the chickens, but you can just add them to the compost pile if you don't have a flock. Try it, your cats will love it!