Best deal would be from your nearest dealer, so you don't have to pay shipping.
dealerlocator.evinrude.com/EN/?Rnd=1186411944609
Iboats should have the Aftermarket Sierra 18-2913.
http://www.iboats.com/18_2913_3_Cyl_...view_id.173842
The OEM part # has been superseded to 331916.