The best olive oil is from Jaen, but there are other places where it is superb: in general all the south of Spain. Of the places you are going to stay I will try in Seville and Madrid (it's the capital, so you can find everything here).

Where to buy it? I recommend to try in a cooperative of producers. They have the best, most pure, most tasteful and cheapest oil. I've buyed it myself and I can assure you it was like eating fruit. Try to put some olive oil on a toast in which you previously passed a tomato or a garlic, great!

I think I should post somewhere in the message board some spanish food recipes, don't you think? :-P