A couple of things that you could do is to enter the work 'Spanish' on the Autotrader and eBay cars sites.
There are a couple of garages in the UK who specialise in left hand drive cars and some are still foreign registered. It is possible to check the full status of a car on a spanish government web site. The dealer could help with this, as would an agent in Spain who registers cars.
Ebay has car on it that are on sale in Spain so it may be possible to line up a vehicle before you go out to Spain. Some of the advertisers are genuine Spanish dealers.
But Buyer Beware, don't pay the money until you touched the metal of the car and shook the hand of the owner. Never go outside the web site or sent money via a Money transfer company, and bank account transfers also cannot be easily reversed.