http://www.mercruiserparts.com/Show_pictures3.asp?dnbr= 75471&ivar=IMAGES/CRUISER/75471/18.png&inbr=1592&bnbr=80&bdesc=HEAT+EX CHANGER+AND+COOLANT+RECOVERY+ASSEMBLY
Sorry you can only by the whole recovery bottle to get the cap :( here but this site might be a step in the right direction.