I buy authentic Drinkwell filters (a 3-pak) on eBay from a user whose handle
is 'debnroo'. It's cheaper than buying them at PetCo or Petsmart and
they've always offered free shipping.

Oh, and Kajikit! Pay particular attention to the instructions about
removing the motor and cleaning it when you clean the fountain (I clean my
fountain once a week). Otherwise it can burn out and then you really
*would* have a $35 cat toy. You can buy a cheap long skinny brush wherever
they sell aquarium supplies. A small straight toothbrush will also work.
And don't forget to remove the "impeller" (the little propeller thing).
It's held in place by a magnet and pulls out easily. You need to clean the
inside of the motor where the impeller fits; that's where it gets gunked up.

Jill