Pet Mart has locations all over. They have good stock of cat foods and suplies. You can get a discount card there.
Another good pet shop with good stock of cat foods and has even better prices than Pet Mart is on the Doi Saket Road (actually Kaew Narawat), if you make the u-turn just before the Super Highway and look to your left, you will see a large pet shop. Not Namping, their prices are not so good, but just 200 meters or so before it. I just can't think of the name. They have discount cards too.
I've always found that for expensive cat foods like Royal Canin, that the best prices are on Shopee or Lazada