You can probably find these on the secondary markets. The special babies were all megas and I saw a lot of those for sale.
Regarding collars, yes you can, you just need a collar kit from the mainstore. They used to be in the lobby bit I'm not sure but you can ask a CSR when you are there if you can't find one.
The kits come with full instructions on how to make a collar. To get your cat to wear one you simply rez it next to your cat and touch it then hit yes when asked if you want to apply to the cat. Check its the right cat of course
All cats have a wardrobe click your cat then prims>clothes, if your cat has items applied you can see them here and show or hide them. To permanently remove anitem you need scissors which I believe are in the customer pavillion at the mainstore.
You can also make hats for your cats with kits from the store.
Many secondary markets have accessories stalls with collars and hats made by other breeders that you can purchase if you don't fancy making your own.
Hope this helps