Write to Clare at Sylvan Models and encourage him to do his resin-cast HO marine buoy in O scale.
I, too, have a bit of waterfront as part of my layout, and I asked Clare to put his excellent HO buoy on steroids for us. This was some time ago, and he said he'd consider it. BTW, Sylvan makes a dandy tug and barge in O scale, both of which I have completed and are destined to be placed in my waterfront.
Otherwise, see the other responses....or put on our 'scratchbuilder's' hat!
KD