yes there are a few, very effective too, espeically when on an opposite bank. the best ive seen however have been home made job, small silencer off a car strapped back under the bike with flexiable exhast hose connecting it.
http://www.cleanacres.co.uk/atv's.htm try this, see exhaust silencers.... or ring and ask for steve. (honda)
http://www.quadshop.com/index.asp or try these and ask for Matt.
pm me if you are going to speak to them