For kitten food, I use the subscribe-and-save option on Amazon. Since it's pretty predictable the frequency you have to buy food, I have it set to send at a particular increment. It's free shipping, and you get a pretty good discount by having it on this option. Also, the time for delivery (if you need it earlier/later) is all flexible. For my older cat's prescription food, I use the autoship version on petfooddirect.com. Again, it's free shipping and additional discount for using the autoship. Besides, it's great to not have to carry cans of cat food up a flight of stairs! A great place to get very, very durable scratch post is, of all places, sisalrugs.com, a retailer of sisal rugs. Their cat products are really well made and very affordable.
I used to buy my pet food at a specialty pet store(Feeders Supply or Petsmart). We recently switched pet food. Now we usually buy it at Rural King which is a farm/feed store. It really depends on which store is running a sale that determines where the pet food is cheapest.
