Yeah, weird! They used to be all over the place, different styles- some had glass, some were like a cage... Now when I do a search all I find is ones that are out of stock or plans to make your own. I was thinking that if you could find a used cage (a bird, small animal or dog crate would work) with the bottom dimensions about the same size as your window opening, you could make a "floor" for it our of 1/4- 1/2" plywood and make one that way. All you'd have to do is remove the bottom of the cage (and wire the doors shut), place the opening of the bottom against the open window, secure it to the window frame, then put the wood piece (cut to size) as a new floor...
I may not be describing it very well, but it should be easy to do.
