Drinkwell Outdoor Carbon & Foam Replacement Filter Combo Pack
I just needed to buy the foam pre-filter but I couldn't find just that. Instead they make you order the whole kit to get the foam. I use the Premium Replacement Charcoal Filters so don't want to pay for the whole kit. It's a good value if you need the 3 charcoal filters but I just wish I could buy the foam by itself.