Try EYE Marine http://www.mooringequipment.co.uk/ They also trade as Boat Gear Direct for retail customers, but their web site seems to be down at the moment. I have bought heavy chain and shackles from them in the past. I'm sure they have a selection of bouys.
Just one question: I was always of the mind that buoys were perfect spheres - but these all seem more pear- or tear-shaped in section. Any observations?