I like a reasonably heavy pin cushion so wouldn't use fiberfill, etc. I can't remember if I used walnut shells or lizard litter; both were recommended when I was taking a class...whatever it was came from the pet store.
Since then I've heard that silica gel (used for drying flowers) is good in pincushions. Have the stuff, but haven't used it yet.
A friend bought play sand at Lowe's and used it in her pin cushion...the kind that attaches to a scrap holder that sits beside your machine and hangs down in front of your table. That pin cushion needed to be reasonably heavy; she just used regular quilting-type cotton fabric.
Good luck!