You can use Peaches & Cream Cotton Yarn from Walmart or grab Cotton Balls from the cosmetics aisle. I don't know of any vendors that specifically sell cotton wick, it's more of a DIY setup. You would (gently) pull the silica from within the coil head and then (gently) twist cotton through the coils.
You'll want to slightly moisten the end of the cotton your threading through to make sure it goes through easily and then just trim both of the ends down to the size of the silica that was in there before, which is right at the ring.
It sounds complicated, but it's pull, thread and trim. Then prime the cotton by dripping a few drops to get it wet, put the post back on and then screw it back into the base. Let it set 1-2 minutes and vape away.