I, for one, do not have easy access to poly stuffing, so I was wondering, what is the second best choice for amigurumi stuffing?
Cotton Balls - try pulling them apart a little with your fingers to give them more volume & squish.
Old Pillows - cut open an old pillow you don't need for the fluff inside (which is more or less polyfill).
Other possibilities: Roving wool, bulky/super bulky yarn (loosely wound up), microbeads, beans (don't get them wet though!).
Arne and Carlos have a video on how to recycle little bits of leftover yarn and using it as stuffing: https://www.youtube.com/watch?v=kogj9fSNqv4
They demonstrated brushing small amounts of leftover yarn back and forth between two carders (big wire brushes) until the fibers came apart and softened and it all turned into a fluffy blob of soft material suitable for use as stuffing.
They recommend using wool and said that cotton is too hard. No mention was made of acrylic or nylon or any other materials that yarn can be made of. I would think that any fiber would soften and fluff if worked enough, though.
Also, you don't have to buy official carders to do this. Pet brushes would work fine.