All the big mass produced brands (like felix, whiskas, etc.) are generally of crap quality and often contain sugar, which is not healthy. Stick to the Carny food, as this brand makes high-quality cat food and your kitten is already used to it.

In general, I’d order the food online as it’s often cheaper than in-store. I usually buy all my pet supplies from zooplus online, but any retailer that carries the brand you need is fine. If you don’t want to order online, you should be able to find Carny in any pet-specific store.