I get a lot of questions from people about refilling their bean bag chair. You can buy filler from lots of sites on the Internet and it will cost you close to the cost of buying a new bean bag chair. You can buy recycled or virgin polystyrene - I recommend recycled because it is better for the environment. The newer Fuf chairs are filled with foam so that is also an option.
I think the most inventive and creative thing to do is to create your own filling. I have read some discussions where they suggest using actual beans in their bean bag chair. Makes some sense but make sure you have a waterproof liner because you don't want your bean bag chair to sprout. Even though beans are a cheap alternative they aren't overly comfy. I like the idea of recycling what you have in your home already.
If you storage room is anything like mine, you have empty boxes full of polystyrene packing materials. You likely have some old camping foamies hanging around in the shed. Spend some time crushing the polystyrene - put it in a bag and stomp all over it. Good stress reducer and some fun, all at the same time. Shred up the foam either with your hands or scissors. Combine the two and you would have pretty comfy fill for your bean bag chair.
If your storage room is nothing like mine and you have none of these materials at home you can find them inexpensively in your town. Look for a foam shop and buy their scrap pieces, they often sell bags of scrap pieces for little cost. Phone around to appliance shops and ask when they are uncrating their next shipment, show up and you can help yourself to their polystyrene packing.
The fastest and easiest way to refill your bean bag chair is to buy it pre-made on the Internet. The funnest and most satisfying way is to make the filling yourself. You will have to experiment a little bit but that is part of the fun.