Go find yourself some fatty bacon and suet at the grocery store.
Cook the bacon down to get as much grease as you can and add in some suet for more volume.
Get some old cans or other non-glass container and pour the grease in (after it's not scolding hot) and either just let it sit, or put it in the fridge.
It will all harden back up (all sat fat!) and when you're ready to use some, you can just get a few tablespoons or 1/4 cup or whatever and 'melt' it in a small pan then pour it on the dogs food.
You're just rendering the fat and storing it for later.
Think of it like a tub of margerine. (literally a tub of lard).