I use a nice biological extra virgin olive oil (1st cold press) which has an acidity of 0.2%. It's called "Alfandagh Azeite Virgem Extra Biologico". I buy it in the Continente Supermarket.
Going to the producer's website (an old family owned company) I see they are in Alfândega da Fé (which is in the Alto Trás-Os-Montes region). This is North/East of the Douro valley and if you have a car and you're an early riser, you may be able to include a visit. Visit the Douro Valley until Pinhão and from there it's a 1h15 min drive. Driving back to Porto through Vila Real takes about 2h15.
Here's their website: http://www.casaaragao.eu
And if you like, have a look at Google Maps for their location.
As August is a holiday month I suggest you contact them to see if they are open for visits.