Where can I get fish for my Ocelot in a superflat world?

Solution 1:

There are no oceans in the basic superflat world, but you don't need to feed it like minecraft dogs, who need food to restore health. In other words, it will never starve. You couldn't make it grow though.

Solution 2:

You don't need an ocean. If you can get have two buckets worth of water, you can create an arbitrarily big body of water yourself, just dig it (or build embankments) and use the water source duplication techniques to fill it.

Given sticks (in case of lack of trees obtainable as drops from witches) and string (obtainable from spiders) you can craft a fishing rod, and use it to catch fish in your artificial pond. No fish mobs will spawn, but you'll still be able to catch 'generated' fish through the old (pre 1.13) mechanics.