The best place would be your local Mercury/Mariner dealership.
The cheapest place? Where you bought your motor from would be a good start, assuming it was new. Dealer margins on a prop are normally 20-25%, you can normally split the profit with a friendly supplier!
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It is better to remain silent and be thought a fool, than to speak and remove all doubt!
Why don't you try to get one from the US. Possibly it will be 25% cheaper than what it is in the UK. Try this link, ask them for what you want and will reply to you with price, they can also find one 2nd hand and warranted too. The shippinf cost will not be that much is you don't do it FEDEX. They are pretty good and have extremely competitive prices in many parts http://www.midtnmarine.com/
