Some Tumblr blogs are dedicated to Grey's Anatomy fanfiction. You can search for tags related to Mark Sloan and OC Wife on Tumblr. There are often smaller, more niche fanfictions shared on Tumblr, and sometimes the authors post exclusive content there. It can be a great source if you want to find something a bit more unique or if you're interested in following a particular author's work.
Fanfiction.net is also a good place to search. There are many different categories and tags you can use to narrow down your search. Look for the Grey's Anatomy section and then search specifically for Mark Sloan OC Wife stories. However, the quality of stories can vary, so you might need to sift through a few to find the really good ones.