All the sites we have contacts with here seem to be for sales and bulk property uploads, have you looked at expat websites / forums etc?
Here are a few to have a look at:
www.spainexpat.com www.expatfocus.com www.britishexpats.com www.expats.org.uk www.expatforum.com www.expats.com www.euroresidentes.com www.expatica.com
There are always people looking for long term accommodation on these sites, I have done the same when I was looking for somewhere to stay in Greece.
Otherwise try some of the rental companies on the Costa Blanca north, but I can't find anywhere which is free to advertise that is any good.
Like I said, maybe Colin, Homefinder or Matt will know of somewhere that is half decent to place an advert.
