i run a 2-into-2 Zard undertail on my S2R800. you can't do a "true" undertail, as you need to leave room for your wheel/tire in there.
it's not a cheap system by any means... a new one would run you around $1800
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i believe. the install is relatively straight forward, did require some exhaust sealant on the header pipe ring gaskets. The only reason I have this exhaust is because I found it used for about $600, which was a hell of a good deal. They don't seem to pop up used very much anymore.
here's a shot of my bike, there's a pipe on each side...
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our great sponsor MonsterParts appears to have them in stock...
http://www.monsterparts.com/pc/ZD017SKR/Exhaust/ZD017SKR.htmlif you're just looking for some slipons, the marving kit looks nice.
http://www.monsterparts.com/pc/DMAS4R/Exhaust/DMAS4R.htmlyou could also piece together a full spark exhaust from motowheels. just get the slipons you want (carbon, dark, or titanium) and a midpipe for your bike. should be around $1200... they have the carbon cans up for $100 off right now.
http://motowheels.com/search.html?q=spark+exhaust+s2r&go=Searchhope this helps!