Hi Helcat
I'm just back from Spain myself. I booked my car with ArgusCarHire.com. It cost just a little over €100 for the week, pretty cheap when you work out the cost per day! Anyway, I've gone back and looked at their payment methods section. They accept Mastercard, Visa, Visa Delta, Visa Electron and Maestro/Laser cards. Maybe you could give them a try.