Fanfiction.net might also be a good place. Although it has a wide range of fan fictions, you need to be a bit more patient when searching for the specific 'Remus and Sirius Soulmark' ones. Look through different categories and use relevant keywords in the search bar.
Some Tumblr blogs dedicated to Harry Potter fan fictions might have some 'Remus and Sirius Soulmark' fan fictions shared. You can search for relevant tags like #RemusandSiriusSoulmark or #HPFanfic to find them.
One great 'Remus and Sirius Soulmark' fan fiction is 'The Mark of Their Bond' which beautifully explores their relationship through the concept of soulmarks. It delves deep into their emotions and how the soulmark affects their connection.
Some Tumblr blogs are dedicated to sharing and discussing Sirius and Remus fanfiction. You can search for relevant tags on Tumblr as well, like '#siriusandremusfic' and you might find some great recommendations from the fandom community there.
Fanfiction.net is also a good place. There are many Harry Potter fanfictions there. Although the interface might not be as modern as AO3, you can still find some great 'Harry is Remus and Sirius cub' fanfictions if you search carefully. Try different combinations of the words in the search bar.
You can try looking on Archive of Our Own (AO3). It has a large collection of fanfiction, including Remus and Sirius stories. Just use the search function and type in their names along with 'lemon' if you're specifically seeking that genre.
Another option is to check out Tumblr. There are many fanfic writers who post their works there or at least share links to their fanfics on other platforms. You can search for relevant tags like #remusxsiriusfanfic or #wolfstarfanfic (as their ship name is often referred to as Wolfstar) to discover some great stories.
Fanfiction.net is also a good place to start. There are thousands of Harry Potter fanfictions on there. However, the search functionality might not be as advanced as AO3. You may need to dig a bit deeper. Look for tags related to the relationship you're interested in. Another option could be Wattpad, although it has a more mixed bag of content quality, but still might have what you're looking for.
You can try Archive of Our Own (AO3). It has a large collection of fanfiction, including many works about Remus Lupin and Sirius Black's relationship. The site has a great search function that allows you to filter by tags like 'Remus Lupin/Sirius Black' and other relevant keywords.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you can search for 'sirius oc remus threesome' there. Just be aware of the site's terms and rating system.
You could also follow some fanfiction writers who are known for writing about the Harry Potter universe. Check their works as they might have written something related to Sirius hurting Remus.
Some fanfic review blogs also help. They review and rate different fanfics. Look for ones that specifically cover Harry Potter fanfics. These blogs can give you a good idea of which Remus Sirius fanfics are worth reading based on the plot, writing quality, and how well the characters are portrayed.