Okay, let's be real, you're heading into some pretty specific corners of the archives with that ask. I spend way too much time in the Snape/Harry tag, especially the weirder alleys of it, so I can point you in a few directions, though the mpreg layer really narrows it.
Your absolute top-tier spot is Archive of Our Own. The tagging system is your best friend here. You'll want to start with the 'Harry Potter/Severus Snape' relationship tag, then filter from there. Click on 'Harry Potter/Severus Snape' and use the 'Filter' button. In the 'Additional Tags' field, type 'Mpreg' and 'Forced Marriage'. Sometimes the 'Arranged Marriage' tag gets used too, so maybe include that. The real trick is sorting by bookmarks or kudos—the quality in this niche can be a real mixed bag, and high engagement usually means someone actually managed to make the premise work. You might also need to filter out 'Underage' depending on when the fic is set, which AO3 lets you do pretty cleanly.
Beyond that, I've had some luck on specific LiveJournal communities that are now basically archives, like 'snapepotter'—searching there with Google using 'site:community.livejournal.com/snapepotter mpreg marriage' can unearth some ancient, oddly charming fics from the mid-2000s. The writing style from that era can feel dated, but there's a certain raw commitment to the concept you don't always see now. FanFiction.net is practically a desert for this specific combo because of their content purges years back; anything explicit or even heavily implied won't survive. A lot of authors migrated their stuff to AO3, so you might find notes about that. Honestly, half the hunt is following authors who write one dynamic you like—if someone does good forced proximity Snape/Harry, check their bookmarks for mpreg stuff, it's a chain reaction.
Be prepared for a certain... tonal whiplash. One fic will be this intense, dark political thriller with the marriage as a sinister contract, and the next will be pure absurd crack where the mpreg is played for laughs. I kind of live for that unpredictability. Just remember to check the author's tags and warnings like your sanity depends on it, because in some of these stories, it genuinely might.