That's a Crank Bros. Speed Lever. http://www.crankbrothers.com/speedlever.php
Many shops carry them, or can order one. Most on-line sellers also carry them. I always carry one, you never know when you will ask someone if they need help on a ride, only to discover that this is what is needed. Also is good insurance on a long solo ride when your hands may be weaker than normal.