Hey,
just as every divine: They were offered during an event. You can get them if they return in some way like
* currently, the ("black") friday sales - where Zebra is not a part of this year
* from luck items during a weekend promo
* from the olymp, if they show up there
* from packs (for real money), if they are in there of if there's a divine of your choice offered (even though I wouldn't recommend picking Zebra there ^^ )

and, Zebras happen to be some of the few divines which we can sell, so you may find some in the sales (I just found some, pretty cheap actually - sort for breed: divine)

~ c(=Mieez

I forgot to mention:
sort for breed: divine and by lowest price

Thank you so much cheers