You should check out some Facebook groups. Usually, you can find the best offers there.
Casas/Quartos para arrendar no Porto
ERASMUS Porto 2018/2019 by student-travelers
ERASMUS Porto 2019/2020 by ERASMUS Madrid
Procuro Casa / Apartamento para alugar em Vila Nova de Gaia
ERASMUS Porto 2018/2019 #1
ERASMUS Porto 2018/2019 #2
Aluguer de Quartos e Apartamentos – Porto
Porto – Quartos e Apartamentos
ERASMUS Porto 2019/2020
Arrendar Quartos Porto, Lisboa e Coimbra – Erasmus Rooms and Accommodation
ERASMUS Life Porto 2019/2020
Casas / Quartos para arrendar no Porto até 500€
ALUGAR QUARTOS / APARTAMENTOS NO PORTO
Quartos/Casas para Alugar no Porto
Rooms to rent / Quartos para arrendar no PORTO
Porto Rooms, Apartments, Flats & Coliving Accommodation Rentals
Alugar/Rent Apartment – Porto
ERASMUS Porto 2018/2019 by ESN Porto
Rooms for ERASMUS @ Porto’s downtown