Wag.com does 15% off for 3 months for new customers. A couple of months ago they had a deal for 30% off of Nutro products so I keep checking them to see if they run it again.
I order from Amazon with subscribe and save. If you do 5 items on subscribe and save you can get 15% off every order. They don't have a lot of cat food listed for the deal but they do have Natural Choice and Natures Variety so I'm able to find 5 items in order to get the 15% off.