Donostia - San Sebastián has a wide range of accomodation alternatives, from 5-star hotels to guest houses and bed-and-breakfasts.
As prices and availabilities change on daily basis our best advice is to check of the following links.
Trivago is probably the best accomodation information aggregator available. Search for Donostia once there.
Another intersting links:
San Sebastian Reservas
San Sebastian Turismo