DE is Diatomaceous Earth. It's a fine white powdery substance. Most pool stores will carry it. Just call and ask for "DE" or "DE filter medium". Both the pool stores in my town didn't carry it, so I had to call stores in the next town over to find any. I paid $14 for a 20 pound bag...it was the only size bag they had. That amount will last me for YEARS (since I'm just adding a little at a time to my sand filter).