www.nitrodesk.com, or search in the market for "TouchDown". Please note, there are three (potentially confusing) versions. If you are on a recent phone, first install "Exchange by TouchDown", try it for a few days, then purchase "Exchange by TouchDown Key" if you decice to purchase.
If you are on an older phone (android 1.1) without Cupcake or Donut, you can download the "TouchDown Non Cupcake" application instead of the first one.
if you have issues, pls email support at nitrodesk.com