some sites do sell the cells to replace for the batteries.
i believe someone on this forum created a thread stating he did replace the cells and it worked fine. I think he also posted a site on where to buy them.
I would think that it would be easier to just replace the battery and get all new cells. That way you don't mix new cells with older, and possibly weak cells. But i'm no battery expert, and am to lazy to take the time to replace cells.........