Hey Stu,
We're in the same boat, hopefully have something lined up for the beginning of September, but have been monitoring listings since about this time last year:
Iberpisos.com - seems to be the online version of the diario de ibiza, but hasn't had any new listings for ages, so make sure to also check the paper when in Ibiza.
Idealista.com - nationwide rentals/sales, has quite a few listings for Ibiza, again not much come on recently.
Fotocasa.es - not sure how this one works, may just crawl all property listings and so be unreliable?
Other than these, really just have to go through all the agency websites, as they don't seem to list on a central site like rightmove etc. Google for 'ibiza inmobiliaria' and get bookmarking. Some of the better ones I have found:
ibizapropertyagents.com - English owner? Very helpful
ibizapropertysearch.com - only 1 listing atm, had about 20 last October
ev-ibiza.com - pricey but good properties
All the sites will have many more properties listed in sept/oct, then reduce through to March/April when there wil be pretty much nothing left, but if you see anything now that you like may be worth contacting. The property we are hopefully getting has been listed for about 10 months and when I contacted the owner in June was told that it has holiday lets through the summer, but is available from the beginning of September for long-term rent.
Seems most agencies charge 1 month - 6 weeks agency fee, plus deposits of more than 1 month seem common (unless others know otherwise - we've been asked for 3 months as deposit).
Emails/messages in Spanish get more responses, obviously, I just used an online translation tool as it's only a couple of sentences you will be sending.
