Papaya, there's a specialty store on Po Hing Fong street in Sheung Wan. Very close to Dear Dogs and Cats, so worth walking down the Pound ln. stairs (turn right on PHF, it's a few shops down near the bakery).
But I have to say, while I don't always like their customer service, WnP seems to have really good quality products. Their cat toys are the best, and they're the only supplier of my preferred cat litter. I think they would be your best shot.
With WisePet and Epet online they have very good prices for food (better than WnP), and for big items they will normally accept returns! Just ask them, and then split up some orders of food to get items and then potentially return w/free delivery (if you order food again ).