I don't know where the place is that you are looking for, but the best place I have found for booking hotels or holidays independently is on the internet.

http://www.hotelopia.co.uk
http://www.expedia.co.uk
are a couple of examples.

Quite a few of the major travel agencies have web sites you can book with and often are discounted, such as: http://www.thomson.co.uk
There are many holiday sites you can use, so it's worth doing a search. I have used the above sites to book hotels and holidays and they are pretty user-friendly. I don't know if they have the hotel you're looking for on them, but it's worth a look.
There's alway's the old-fashioned way of going into a travel agency!

Thank you for the reply. Port De La Selva is just outside Roses on the North Eastern tip of Spain. It is a tiny fishing village and that is why no travel agents go there. Thanks for trying anyway.

It might still be worth looking on the internet as there are a vast array of sites that you can book hotels on, not necessarily just travel agency websites.
Have you searched to see if the hotel itself has a website? Either that or you could ring them.

Hope you suceed, it's a pretty long way to go and find out they are fully booked when you get there!

There are some apartments on this site:-

Accommodation in Port De La Selva

Wow. Thank you. I have been searching for ages on appartments in Port De La Selva and haven't come with anything as good as that.