Boating is a huge industry these days and alongside with that the boating accessory business is also big. There are plenty of manufacturers and dealers of boating accessories around.
A boating accessory is usually sold by boating equipment suppliers who are in the UK and Australia also known as chandlers. If you live in the United Kingdom you can find a good list of chandlers at http://www.ukcanals.net/chandlers.html. There is listed about 15 different sellers of boating accessories.
At http://www.metstrade.com/marketplace/azbrowse.asp there is a very comprehensive list of companies that will sell you a boating accessory. They are listed with a country code and there are countries from all around the world represented so finding one that will work for you should not be a problem. The list is huge though, but offers a good search option.
If you live in the US you may wish to visit the Boat Owners Association of The United States. They have a large list of manufacturers and suppliers of boating accessories at http://www.boatus.com/links/mslinks.asp. At their site you are also able to find advice for buying your boat, insurance help, message boards and much more that can help make your boating experience a great one.
Yet another place to shop online for a boating accessory is at Bart