Before the question can be answered, we'll need to know how the cushion will be used. Indoors? Outdoors? Both? Will it get a lot of wear or a little? Will it get wet or will you need to wash it? Will it be for seating or just decorative?
The best place to look for cushion filling materials (pillow forms, foam, fiber fill, styrofoam beads, etc.) would be a store that specializes in Home Dec. They should have exactly what you need plus the right kind of fabric for the job.