Auto Trader have a parts & salvage site:-
http://www.autopartstrader.co.uk/
Most of those offering parts include a courier service (for a fee), so you could find your required part from anywhere in the country.
But like ebay, if having the part is not time critical, you can wait and search for the part at the right price. If you are not fitting the part yourself, many garages will offer to search their contacts (for the second hand part), knowing that they will get work for repairing your car.