TailLight
For a simple clear lens swap, searching on Google, eBay or Amazon is the best way to find these. They are available from Clear Alternatives and BikeMaster, and are available in both early (1989 to 2000) and later (2001+) styles. Search for "GS500 clear tail lens"
If you're looking for the smoked lens look or a tail light with integrated signals and LED's, eBay and Amazon is again the way to go. It should be noted that these are only listed for 2001+ GS500's... BUT... that later lens style CAN fit with the older GS plastics (it's just really snug).
LED Tail Light Bulbs
If you're happy with how your tail light lens looks but still want to upgrade to LED's, there are direct replacement 1157 tail light bulbs available. LED's have the advantage of being longer lasting, shock proof, faster response time, brighter, and they put less strain on the GS's charging system.
A search on superbrightleds.com, eBay or Amazon for "1157 LED brake bulb" or "1157 LED tail light bulb" should turn up some good options.