Here are a few good places to start. Can I ask if you have a Dane or are experienced in the breed? I have two and they are the loves of my doggie life. Anything you need to know just ask :0)
The best one to contact would be the second link, they may sound far a way but have home checkers country wide.
The thing being, many people can get suckered in by ads on the likes of Preloved and Loot etc but if they go through a dedicated rescue they will be getting a life time of support if anything should go wrong. You have no such back up with a free ad in the paper or on line....they really wouldn't know what they were getting.