Sponge/Cushion for seat

I am planning to overhaul my dining chairs made of straw from Ikea.
Basically like this

http://hautekhuuture.blogspot.com/2011/02/i-spy-diy-ikea-dining-chair.html

Can someone please tell me where I can find free/cheap cushions for this? I tried the shops and it is a bit too expensive for me.

Thanks
Sri