Cocklebury farm on the A259 just past the pub on the left at Bosham.
Or buy a new "Volvo" from superstart at Southbourne - the guy in there is a boaty person and very helpful.
Take it to a motor electrical repairers. I've used one locally, they do a complete repair and overhaul for about 50 quid. There's nothing special about it being a 'marine' alternator, so don't waste your money going to a 'marine' repairer.
Thanks Ian - I have sourced a replacement (delivered today from Superstart.)
Thanks Cantata - I would like to get the old one rebuilt as a spare. I have to have a quick turnaround for a replacement as the boat is out again on Friday.