"I would like for Petco to cater to every animal, not just a select few."

That's a silly statement. Petco hardly caters to a mere "select few" and does provide products for a variety of the most common household pets. And really? You want Petco to cater to every animal? That'd be a hell of a big store.

At any rate, I can't imagine acquiring a pet of any kind, let alone a relatively newly domesticated animal who seems to be a little high maintenance in a few areas, without already having the basics ready to go. Perhaps if you'd gotten prepared beforehand, instead of a complaint letter to a store that didn't do anything wrong, you'd be writing a letter complimenting another pet store for having all the supplies you need.