I've spent exorbitant amounts of money on catnip sprays, catnip mice, dried catnip herb itself but not one of my cats has ever reacted to it!!
The only thing they seem to like is Kookamunga Catnip treats, but it's probably because it's tuna flavoured.
I guess the problem is that I adopt cats from around my neighbourhood and they're all from the same gene pool that is not pre-disposed to react to catnip. It's a damn pity though, I'd LOVE to see my kitties stoned : P