I was hunting for those exact stories a few months back and hit a real wall. Most free sites felt either super sketchy or were just filled with fanfic archives that didn't organize things well for original fiction. My breakthrough was actually a mix of things. Subscription services like Scribd have tons of stuff in their catalogs, and you can often get a decent free trial period to binge-read a few. I also discovered that some indie authors publish their mpreg works serially on platforms like Tapas or Radish; you earn free 'passes' daily to unlock chapters, so if you're patient, you can read the whole thing without paying a cent. It's slower, but totally legal and supports the writer through ad views.
Don't sleep on library apps either—Libby and Hoopla are lifesavers. You link your local library card, and the selection is shockingly good sometimes. I found a whole paranormal mpreg series there that I'd seen for sale on Amazon. The waitlists can be long for popular titles, but putting a bunch on hold means they trickle in over time. It feels less like a targeted search and more like a pleasant surprise when one becomes available. The quality on official platforms is just more consistent, and you avoid those pop-up hellscapes.