I KNOW I've seen it in the cash shop and elsewhere, but shoot if I can't find the item/link/etc now. Can someone kindly point me in the right direction?

Two places: For CSC: Personal -> Pets -> You are using X of your X pet slots. Buy more? For sP: Subeta -> Explore -> Centropolis -> Millionaire Center -> Pet slots

AHA! Oh thank you, I KNEW it was going to be super obvious!

they can also be obtained by a loyalty totem from your 2nd yr up onsite via loyalty boxes

and sometimes awarded via things like the quest-a-thon