They come up on eBay every so often but usually sell for close to what brand new systems go for. You get a used system for $250 to $500 less than a new set . . . . but you don't have to wait on them
I followed them for a year or so looking for a set for my bike and I've seen several nice sets sell there. One set with a small dent in one of the mufflers went for $1500 and a perfect set sold for $1700 if I recall correctly. New, chromed steel (all of the mufflers are stainless) sets go for about $2K . . . . but you may have to wait a spell - Jan only gets a delivery or two per year and many of them are already spoken for before he gets them. There is also a 100% stainless system available by special order for about $500 more
The Gazelle pipes I've got for mine are 6 into 2 into 6 type, I may put a small curve extention pipe before the mufflers to have them pointing more upwards at the back when they are fitted......