In the UK Wills Marine who, are Mercury agents, would be one of my top choices. http://www.willsmarine.co.uk/
Good reports on here of the service from them.
Nearer to home I would try Ron Hale in Portsmouth although I have no past experience of dealing with them. http://www.ronhalemarine.co.uk/
BUT look at the Mercury Marine website for dealers in the country where you want to buy the parts. ( Portugal?)
Try s/h dealers for a used part. I bet the Mercury one will be big bucks. Wills Marine will supply it as a Tohatsu part (same but cheaper) if you need to buy new.