Sorry, never seen anything like this.  Why not consider making your own.  Creating forms is not difficult and you will find lots of discussion on this site.  Carvable foam is one option but there are many others.
DavidO

You might try checking out the toy section of your local stores.  Sometimes you can find large, rubber toy fish (amidst dinosaurs, farm animals, reptiles, insects, etc.) that might do the trick, depending on the scale you want.

The toys are a good idea. I did find this one online, they would probably look good just as they are on the tree LOL, but you maybe could use one as a model to mold different versions?
http://www.perpetualkid.com/index.asp?P … rodID=2542

Also look in places that sell cheap ceramics, even dollar stores or Asian markets.

Using a firm mold shouldn't be very difficult:  wrap it in several layers of paper/glue strips and smooth well, then use a craft knife to cut in half (I would cut lengthwise over the top of the head, down the back to the tail, then the same direction down the belly).  Then glue the two halves together with more strips.

If you could find a basic fish body shape that was appropriate, you could make fins and tails separate and attach them.  Or get a chunk of Plasticine (kids non-drying) clay and carve your fish body, and add fins and tail.  This way, you could have a straight fish, and curved fish so they aren't all alike.

It sounds like a wonderful tree!  I've seen a lot of traditional trees, but there's always room for something new!

p.s.  Another name for koi is carp.

Sue

Here's another possibility to use as a form for a fish project.  It's a stuffed animal that I picked up at a local thrift store for a mere fifty cents last year (the bottom picture shows my left hand holding it so you can get a sense of the scale). 



While I wouldn't recommend it, if you don't care about ruining the plush critter, you could try applying the PM directly to the fabric surface, but, I think it'd be wiser to wrap it in plastic wrap or something similarly waterproof instead, not only to preserve the form, but to make it much easier to remove the papier mache from it.  Stuffed animals are obviously squishy, so you won't be able to exert a lot of force on it initially, but, once you've built up several layers of PM, and allow them to dry, it should become hard enough for you to apply a fair amount of pressure as you continue to work.  If you added some temporary structural support to the fins, so that they'll stay put/erect (tape some popsicle sticks to them or something similar), you could apply the PM over them too, instead of adding fins later on (otherwise, I'd just tape the fins flat against the body, before covering the whole shebang with plastic, thereby creating a simpler form).  Another benefit with a stuffed animal, versus a rigid form, is that you can alter the shape a bit--say, if you wanted a bend in the tail of your fish for example--by taping it into the position you want before you start applying PM to it. 

Another, much-easier-to-obtain, idea for a form that just occured to me as I write this [and really should have come to mind sooner--K.I.S.S. (Keep It Simple Stupid) are words to live by],  is a flat plastic or glass bottle (maple syrup and salad dressing bottles perhaps--okay, maybe liquor too, but I don't drink).  If you use the broader base of the bottle for the 'head' of your fish, and the thinner top portion for the 'tail', I think it would work well.  You'll have to modify/build up the 'head' region a bit, as the bottom of the bottle is flat (unless you want your fish to look like it swam straight into a wall at high speed that is) which is easily accomplished by crunching up a ball of newsprint into the rough shape you want, taping it to the bottle, and then PM'ing over that too.  Here's a very quick and dirty sketch I drew in Microsoft Paint, with my mouse, to give you a visual representation of how the idea might work/look:



And here's something that I hope you will find inspirational.  It's a sculpture of some Koi that I found on DeviantArt.com some time ago, by an artist that goes by the handle "creaturesfromel".  I don't believe that they were made with papier mache, but they're stylish and the tails are simply breathtaking.



As a bit of a disclaimer, I suppose I should note that I don't use forms much in my own work, other than simple spheres and cylinders--I sculpt almost everything the 'hard' way, from scratch.