I find that reloading the vendor selection screens helps. If you don't see the Reagent you are looking for at first, close the selection, reopen it, and open the Reagents page. Keep doing this until you see what you want.
I found them in water in Unicorn Way and Cyclops Lane, so I would guess they spawn in most water places.
I sure wish the quests would at least give you a "hint" as to where to find these items. I've been searching for hours for parchment, but without knowing if it's sold, a drop, or on the ground, it's very difficult to find. Especially, with hundreds of other people also searching at the same time.. hehe
Look for water. Cat tails are always in water. So, a couple of places in WC that I have found them are in the mote thing in Unicorn Way and the stream in Cyclops where Nolan is. Ultimately, just look for water no matter what world you are in.
I found dozens of cat tails without even really looking for them - just look near water (like behind Zeke in Wizard City, the streams in Cyclops Lane, and the tops of the waterfalls at Four Falls Mill in Triton Avenue).