The one that was included with my kit was kindof cheap, and it broke when i dropped my bike. I soldered it back together and it works now. I would check any local hardware store, I actually bought one from OSH and it was about 2 bucks.
The one from the hardware store? Yeah, it is more durable than the cheap one in my kit. The installation is a little more difficult though. You have to figure out how to mount it. I just have the cheap one from my kit on my bike right now.
I made my own and it turned out really well.
I took an old plastic hand brake lever and removed the handle.
I took a hack saw and squared up the end of the clamp part.
Go to radio shack and pick up a package of momentary push button switches.
I paid $3 for two switches. It comes with the threaded nut on the backside. Drill a hole in the hand brake ( I think mine was about 1/2") Mount the switch in the hole you drilled and run a wire ( I used an old cell phone charger wire) and connect it where the original was connected to. Here's a picture of mine.