You might try re-purposed memory foam mattress pads. I searched around on Amazon, and with a little digging, found some good deals on king size foam pads, and got a read on the firmness of the foam by the reviews. I used two layers of foam, of different firmness, and the end result is a little thicker than the stock C22 cushions, but I like them better than the originals. It was much cheaper than the "foam store" prices - if you want another option.