You can try Archive of Our Own (AO3). It has a vast collection of fanfiction, including many 'dark tony x peter' ones. Just use the search bar and type in the relevant tags.
One popular 'dark tony x peter fanfiction' story is 'The Unseen Bond'. It delves into a complex relationship between Tony and Peter where Tony has some darker motives that are gradually revealed. Another one could be 'Shadows of Stark Tower' which shows a more mysterious and intense side of their relationship in a unique way.
One common theme is the intense protectiveness that Tony shows towards Peter. In many such fanfictions, Tony's possessiveness drives him to keep Peter safe at all costs. He might go to extreme lengths like building advanced suits for Peter with hidden tracking devices to always know his whereabouts.
Tony's character often develops from a more detached mentor to an extremely possessive figure. At first, he might be just guiding Peter in the superhero world, but as the story progresses, his feelings intensify. His possessiveness could be triggered by a threat to Peter's life, which makes him realize how much Peter means to him.
Another possibility is that Doctor Strange could rescue Peter. He has powerful magic and the ability to travel through different dimensions. He might sense that Peter is in danger through his magic and use his powers to find Dark Tony's lair and free Peter. Strange could also use his magic to fight off Dark Tony if he tries to stop the rescue.
One common theme is mentorship. Tony often takes on the role of mentoring Peter in many fanfictions. Another is family - like bonds, with them acting like father and son or brothers.
One common theme is mentorship. Tony often takes on the role of mentoring Peter, guiding him in his superhero journey. This creates a dynamic where there's mutual respect and growth.
Fanfiction.net is also a good place to look for Tony x Peter fanfiction. There are many authors who post their works there. However, the site's interface might be a bit old - fashioned compared to AO3. But it still has a decent amount of stories related to this pairing. You might need to dig a bit deeper by using relevant tags in the search.
A great Tony x Peter fanfiction is 'Peter's Protector'. It shows how Tony is always there to protect Peter, whether it's from villains or from the media trying to expose his identity. 'The Bond of Iron and Web' is another one where their relationship evolves as they face various challenges in the superhero world. It really delves deep into their emotions. Also, 'Stark - Parker Team - Up' is a fun fanfiction that has them teaming up for a series of adventures, with lots of humorous moments as well as some really heart - warming scenes between the two characters.
Mentorship is a very common theme. Tony is often portrayed as guiding Peter in his superhero journey. Another is family, as their relationship can be seen as a father - son or big - brother - little - brother type. And of course, there's the theme of heroism, where they work together to save the day.