Delkevic makes mufflers and complete systems for the GS500.
US site: https://delkevic.com/motorcycles/suzuki/1998-models/gs500e-f-1989-2002/
UK site: https://delkevic.co.uk/s/Suzuki/GS500%20/1998
They can also be found on eBay and amazon!
Black Widow Exhausts also makes mufflers and complete systems for the GS500.
A website called Predator Motorsport has stainless exhausts for the GS500.
The Jardine website no longer lists the GS500 as a search option... but they can still be found on amazon and eBay by searching for "Jardine GS500".
Vance & Hines used to make a full system for the GS500. It had black pipes and a silver wrap on the muffler. It is no longer available on the website BUT you might still be able to find it on eBay by searching "Vance and Hines GS500". At the time this was typed, there were active listings on eBay (April 2018).
Of course, other mufflers from different bikes could be adapted to fit. The stock pipe diameter has been noted to be 45mm but I personally cannot confirm that. You could find one that slides right on... or use an adapter or mid-pipe for successful installation.
It has been suggested several times on the forum to replace rusty OEM exhaust header bolts with studs and nuts. The idea is to wind the stud in and you never have to worry about damaging the threads in the engine ever again, because when you want to remove the exhaust, you remove the nuts on the studs, rather than removing stock bolts.
They are M8-1.25mm. 60mm for the length has been confirmed to work with the stock header pipes.
More information about this can be found here in Showbizwolf's thread: