Short-term apartment rentals Spain

David, when we are looking for apt rentals in the EU, we usually use VRBO.com, and Homelidays.com. We have also had good luck going to the city websites, and looking at their recomendations.

We rented an apartment in Madrid from apartmentsapart.com with good results. We really liked our place.

In Sevilla - Sevilla5 has a wide assortment of apartments. We rented from them last fall and stayed 5 nights. The responses online to inquiries were good and the apartment was as advertised.
I would use them again.

I use vrbo.com for apartment rentals, both in the U.S. and in Europe.