What type of non-foam filler should I use to make a DIY Coop pillow?

I really like the Coop pillows but they're still just a bit too firm. I know that there is some kind of non-foam filling in these pillows, which I think I need a lot of to get the firmness I want. What is the best non-foam filling to get that will decrease the firmness of the pillow? I've tried adding a little bit of the filling that comes in a regular pillow. I'm not sure if I've added too little or if it's just not the right stuff, but it doesn't help at all.