There are also some independent fanfiction blogs. These are often run by passionate fans who focus specifically on certain pairings or aspects of 'Supernatural'. If you search for 'Supernatural Sam and Sarah fanfiction blog' in your search engine, you might stumble upon some great finds. Just be cautious of the reliability of the blogs, but most are just fans sharing their creativity.
Well, Tumblr can be a good place too. There are many fans who post links or even write short fanfiction bits about Sam and Sarah on their blogs. You might have to dig a bit through the 'Supernatural' tags, but it can be rewarding. Also, some Facebook groups dedicated to 'Supernatural' fans might share or discuss such fanfictions.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a vast collection of 'Supernatural' fanfics, and you can simply search for Sam and Sarah pairings. Another option is Fanfiction.net, which also has a lot of stories related to the show. Just be sure to use the search function effectively to narrow down to the ones you like.