You may want to reach out to Ronan McMahon's team of experts at Pangea as they have done a lot of scouting in Spain (albeit much of it on the coast... still, they may have a broker to recommend who does long-term rentals in the San Sebastian/Donostia area and if so, that'll be info worth having):
Email: info@pangearesearchgroup.com
Website: https://pangearesearchgroup.com
Best,
IL Overseas Editor Jess Ramesch