Places to buy cheap cushion inserts?

I got a couple of cushion covers recently that I need inserts for, and I'm wondering where the best place to find cheap ones would be. I checked out Winners, but they didn't have the size I need (16x16 inches). I'm thinking maybe Michaels, but I don't recall seeing stuff like that last time I was in, anyone got any ideas - Downtown or West Broadway, preferably. Thanks!