are you talking about the point? thats the fatty cut of meat on a full brisket. If you mean just the fat cap off of the brisket I believe some butchers will sell that separately. Just ask for beef fat or yes....fat cap.
If you are looking for fat to make sausage or add to deer burger, the soft fat on the top of a brisket is a poor choice. You want hard fat that will melt when cooking. Such as beef suet or pork back fat. The deckle of a brisket (triangular shaped fat) between the flat and point works well. I trim 20 or so brisket a year for catering and have used a lot of brisket fat for processing.