Hi there! Hopefully member
barry might know of one or two - he lives ON the Plaza Santa Ana - and I don't mean on a park bench under several newspapers, either.
Maybe he can
spy some FOR RENT/SALE signs hanging in some windows and could jot down the phone numbers for you to call - don't expect the sellers/landlords to speak English, though. Prepare for that probability.
Out of curiosity, are you IN Madrid now? Also, does it HAVE to be Plaza Santa Ana? Might it be on nearby streets in the same area?
Good luck! Saludos, MadridMan