Where can I find or make a remote wireless light switch?
There are plenty of remote switches on the market, especially if your light is (or can be) plugged into a wall socket.


X10  is probably the best-known standard for home automation
INSTEON is a backward-compatible extension of X10, introduced in 2005
Z-Wave is a pure-play RF control system
There are lots of cheap'n'cheerful generic switches available.  
The Zigbee wireless protocol is also starting to get some traction in home automation, after many years as a hobbyist's curiosity, but commercial devices are still rare and expensive.


One thing you should seriously consider, however: security.  X10 devices - and the cheap'n'cheerful stuff - DO NOT encrypt their control signals, so it's scarily possible that criminals might remotely turn off the lights in front of your house for their own purposes.  Unencrypted wireless signals should ONLY be used for stuff where interference or disruption won't cause any damage.

INSTEON, Z-Wave, and Zigbee were all built with security/encryption in mind; I'm sure none of them will make Fort Knox feel insecure, but they should be at least as hard to crack as your garage door opener, anyway.