I believe true zebra plecos prefer sand or fine gravel and a good amount of caves more than plants, but I could be wrong. What about botia sidthimunki (or Yasuhikotakia sidthimunki, dwarf loach by its other names)? I can find them locally on special order, but most places have never heard or seen them, but that would probably be different elsewhere.
Green Flash, are you specifically just looking for a very small rare fish? Or are we talking about rare fish in general regardless of size.
I guess it depends on the area you live in. Where I am I would consider Rasbora Briggitae pretty rare. I never see them in stores and would love to get some eventually.
There also seems to be a lot of beautiful variation in the killifish family, and I NEVER see them in stores.
Another rarity that comes to mind is the surface dwelling form of the blind cave tetra (the non-blind form with silver scales and red fin pigment). I would love to get my hands on some of those, even though they aren't considered particularly remarkable.