There are various chains of travel agents, with branches in most places. Try looking out for Marsans, a well-reputed and large agent, who also have
THIS webpage on which, I believe, you can book online.
Similarly, another large agent is Halcón Viajes, and their website is
HERE.
You could google others by using
viajes,
agentes,
españa ... but Marsans and Halcón are two of the biggest ones.