perniciouskitty,
I'm sorry to say there isn't a lot of merchandise out there with either of these guys.
Check out the collectibles section of Muppet Central. There are some posters and puzzles and stuff with pictures of Robin and Sweetums on it, but there aren't really any plushes or figures out.
There is one PVC of Sweetums that is available from Walt Disney World, and another PVC from Europe, but thats all that I can think of.
Welcome to MC!