One popular 'Saber and Harry Marriage Fanfic' could be 'A Magical Union' which beautifully blends the magical worlds of Fate/stay night and Harry Potter. It has an interesting plot where Saber and Harry meet due to a magical anomaly. Their relationship develops gradually, with Saber being intrigued by Harry's courage and Harry being fascinated by Saber's noble presence.
Well, 'Fate's Bond with the Boy - Who - Lived' is another great one. In this fanfic, Saber and Harry's marriage is more of a political move in the beginning as they try to unite two powerful magical forces. But as the story progresses, they start to develop real feelings for each other. It shows their journey from being strangers to loving partners quite well.