My large bean bag has gone flat, how can I fill it and what do I use?

You can get huge bags of bean bag refill "beans" from any department store. It should have a zip on the side to open it, but there won't be a pull tab on the zip (so kids don't pull on it and dump beans everywhere) so you'll need to attach something like a paperclip to it.

Lots of great solutions already, but wanted to add MY solve: chopped up foam pipe insulation. I hit Loews and got 6ft medium/small foam for 1$ each...I ended up using 8 to fill a medium bean bag pillow. I cut the foam with regular scissors( , with blisters to show for it), about an inch long. This is the same foam used for those water noodles for the pool.. but cheaper, and not SO firm. I made two pillows. One took 8, the other smaller pillow used 4. The larger pillow I used 1" rings, whole. The smaller pillow, I cut the rings into 1/8s, for less bulge (whole donut rings show thru slightly on heavy cottons). The smaller pillow showed hardly any more bulge than you might see from regular bean bag fill.