How do I get more zebras?

I already have my first Zebra which I got when I built a ship.

This sounds to me very much like you do not have a zebra. You have instead discovered the zebra race for trading. In order to get an actual zebra joining your village, you will need to play in Iron Will mode (no huts/log houses/mansions built). You will know you've done it right when a Zebra shows up as an actual resource on the left section.

Zebras in Iron Will are a free way to get some small amount of Catpower, which is otherwise impossible without kittens. To make successive Iron Will runs a little easier, every time you reset, your maximum number of zebras will be set to your current zebra count, plus one.

I've gone through the code and this is the only line that increases the number of zebras you have:

reset: function(){
  ...
  this.karmaZebras = this.resPool.get("zebras").value + 1;
  ...
}

This would seem to imply you get a single zebra each time you reset.