Patience, frustration, some more patience, a little more frustration and a little more patience followed by another sprinkling of frustration...and that's how you find zebs.
Seriously though I can't account for everyone on here, but some have aquired fry from breeders, others have trawled Fish shops and some have bid for them on places like ebay.
If your wanting to find some, to breed, the best place is to hang out here...

and keep an eye on places like aquabid as well. Another word of advice if youre wanting to breed them, is have enough cash to purchase a group of ten or so...they quite often come up in groups, as I think a lot of people buy groups of them thinking they're going to make a bundle of cash, when it doesn't happen after 6-12 months they sell them wanting their money back.