Well, FanFiction.net is also a good place to start. There are a lot of 'West Wing' fanfictions there. However, the search function might not be as refined as AO3. But still, if you type in relevant keywords like 'Josh and Donna pregnant in West Wing', you may come across some stories. Another option could be Tumblr. Sometimes, Tumblr users share links or even write short fanfictions themselves, so it's worth checking there as well.
There are several places you can look for such fanfiction. Archive of Our Own (AO3) is a great resource as it has a well - organized tagging system. Search for 'West Wing' and then narrow it down with 'Josh Donna pregnant'. Additionally, Reddit can be useful. There are 'West Wing' - related subreddits where fans might discuss or share fanfictions about Josh and Donna being pregnant. You could also try Google. Just type in 'West Wing Josh and Donna pregnant fanfiction' and it might lead you to some blogs or websites that host such stories.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including 'The West Wing'. Just search for 'West Wing Josh Donna pregnant' on the site, and you might find what you're looking for.