Another place could be FanFiction.net. However, the quality and content can vary greatly. Some stories might be more focused on the relationship between Thor and Sif leading up to the pregnancy, while others might deal with the aftermath. You'll have to sift through the search results to find what you're looking for.
You could also check out Tumblr. There are many fanfiction writers and fans who post and recommend fanfictions. Look for relevant tags such as 'Thor Sif' and 'pregnancy' to discover some great reads.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often a wide variety of fanfictions about different characters and scenarios. Just type in 'Thor and Sif pregnant' in the search bar and it should show relevant results.