Hi Pauline,
Try doyouspain............www.doyouspain.com compares most suppliers and you can filter out those to avoid (Firefly) and compare the different options i.e. full/full fuel or full/empty etc
Check the T&C's to make sure you don't get hit for the €25 charge for logistics (them refuelling after you return it). Can be a bit of a minefield just comparing headline rates.
As John says book early, don't pay and recheck weekly and if the price drops cancel and rebook.
Regards
Dave