they appear on random pages in the NC Mall but you can search "pet slot" too. There is a +10 slot version but if you have more slots that would put you OVER the limit, you won't be allowed to buy the 10 slot item at all and can only buy lesser increments to make up the difference. Also the pet slots are IMMEDIATELY activated on the account they are purchased on upon buying, you cannot trade/gift these pet slots like regular items.
(you need an account to see links)