If you are looking for an easy place to buy food for your Neopet, why not try the always-stocked General Store? If you are running short on Neopoints you can also take your pets to the Soup Kitchen for some free soup.
Once you have found food for your Neopet, feeding him or her is easy! Just go to your Inventory and click on the food item you'd like to feed to your Neopet. A pop-up window will then appear. Select "Feed to (your Neopet's name)" from the drop-down menu to feed your Neopet.