I got my information from Rabbit Health in the 21st Century, page 166. It also says, which I should have mentioned, not to feed rabbits [or maybe any other animal] the seeds. Seeds wouldn't be in any catnip you buy, but with growing plants, just don't use the seed heads.
If you don't have plants, if your rabbit likes dried catnip, you could use catnip teabags: tear off string & remove staple, of course. Your rabbit may enjoy playing with &/or eating the bag & it's ok if they do, though a bit messy with dried catnip all over until they eat it up.