Gill's right, I used to grow it too. It's like mint and grows like wildfire. Get some seeds and you're away. Cuttings also take very quickly and easily. I wouldn't spend the money on an adult plant as it's SO easy to grow from seed. You'll soon be making your own catnip toys.
Dry the leaf and flowers in paper bags, then scatter these where you want the cats to sleep, or if you want to watch them go silly (depending on your cats reactions). Mine love it.