Hobie quit manufacturing and selling Hobie Float Cats and parts a long time ago. Over time the Float Cats will start looking like Frankenboats.
I went through my garage, laundry room, pantry, spice rack and medicine cabinet to see what might fit. I found 4 close fits in the medicine cabinet. They don’t fit perfect but it can be forced on and given a quarter turn.
If you are not after the Frankenboat look, you can wipe down the replacement cap with rubbing alcohol and then spray it with flat black. I did this with all of my PVC pipe accessories and they look pretty good.