The itch.io version doesn't support DLC, pets are DLC. So nobody with a itch.io copy of among us is able to buy pets or any other DLC. That's just how itch.io works sadly. You'll need to use the steam key, then buy the pets via steam, and I believe the pets will still show up whilst playing through the itch.io version i think (i can't confirm this lol) so you don't have to use steam to play with the pets.