DE is Diatomaceous Earth and every pool store and many discount houses sell it because one of the two most popular filter types, the DE filters, use it. But the stores sell it in various amounts and the smallest (10lbs) should last you several seasons. It's also good for getting rid of things like ants.

No, the skimmer socks work AMAZINGLY well, particularly in the spring because the pollen gets caught in them and you don't need to backwash so often.