go to local hardware store and get a replacement one there, if NOT sure what size, take the one out of other side and match it up.
WAY cheaper than buying online and paying shipping
UNLESS you have a LONG drive to store, and then the gas your wasting is about the same
SO will be to dealer if its a LONG drive
DOn't know what yr your atv is, so cannot say if its under warranty or not
YOU can try calling your dealer and asking, never hurts,
Maybe, they will mail you a bolt for free, as its a small item?? odds are they have a wide assortment of bolts on hand, and sure there buying for less than you can!
there your options I say!
Bolts vibrate loose, it happens
add some loctite when you install the new one, and check others from time to time, part of PM's on an ATV