Trainz (one of our forum sponsors) appears to have in stock some used aquarium cars in keeping with the season:
EDIT [ sold out as of 10/27/2021; link deleted by me] Lionel 6-36830 Trick or Treat Aquarium Car VG/Box
Lionel 6-29329 O Halloween Peanuts Aquarium Car LN/Box
https://www.trainz.com/collect...-aquarium-car-ln-box
Lionel 6-37097 O Where the Wild Things Are Aquarium Car
https://www.trainz.com/collect...ngs-are-aquarium-car