I don't know of anyone directly with them for sale. You could check the classifieds at www.applesnail.net or a google search may be your best bet. I am *supposed* to be getting in olive nerites, but don't know when and haven't really seen the zebras around. Also check around some of the planted tank specific forums as often there will be inverts listed in their ads.
You should research whether the "Zebra nerite" you're after is freshwater/brackish/saltwater/amphibious, as I've seen different species being sold as "Zebra nerite".
Hey that's me Lol. I will have some more in maybe two weeks? That is unless someone doesn't pay.. =) Find me on aquabid or just e-mail me to get on a waiting list.
My trashipper just called me, I have 15 comming in this week, so after a 5 day quarantine I should have some. Send me a pm on aquabid so it gets to my e-mail