Hi, anyone know where I can buy pet feed

You do not need to buy pet feed, as such.

If you hit the Feed Pets button in your stables it will bring up a popup with the options to either feed the pets in the stable you are currently in, or all your pets across all stables. The popup will show how many pets are present, and the cost to feed. The pets will be fed automatically upon an option being selected and the player hitting the "Feed Pets" button

On a pet's profile page, if a pet is hungry there will be a Feed Pet button under its Lineage button. This can be used to feed a single pet (it can be used to feed a pet that do not belong to you also. This is primarily for feeding pets that are set for breeding but are unconscious from hunger)

Once a player has passed 10k AP for the first time, it unlocks the Stablehand option in the stables. This can be used to hire a stablehand who will autofeed all owned pets until the contract expires (there are several contract options, with longer periods of time having higher costs (lower periods using gold, longer ones involve diamonds)). Once a contract is up you can repeat the process. The player will recieve a notification in their overview to alert them when a contract period finishes.
As the Stablehand contract involves the cost of paying the stablehand, but the overall cost with the stablehand involves a discount as the number of pets rises, it's a better option when one has a large number of pets, but less so with only a small number

ok, thank you