You should look for exhaust tips with LEDs for motorcycles. Those should have an easy way of attaching to the small opening in motorcycle exhaust. Otherwise you will have to put a car exhaust tip, which may require some extra work for fitting.
Exhaust tips (with or without LEDs) for cars are easily available, these are attached around the tip of the exhaust pipe, the hole is a bit large for GS150's exhaust tip (the small pipe in the center of the exhaust opening). So a larger diameter pipe welded to it should work.
McLeod Road or Montgomery Road would be the place to get it.
Edit: The wire connects to the back/parking light.
The photo is for a motorcycle exhaust tip (off the web), but it has a large diameter, not sure if it would fit GS150 without modification.