Note that you can't get that light switch separate from the whole left handlebar switch assembly.
If you find one of those, then you need to cut the wires to the connector plug for the lights and loop them out; then you can use the switch for your auxiliary function
Here are some great accessory switches from Eastern Beaver that would meet your requirements and match nicely with the OEM stuff.
http://www.easternbeaver.com/Main/El.../switches.html
He's in Japan but things come within days of order.
Great guy! Check around the web-site - he has all kinds of useful electrical stuff on there that is all bike-related