That's a very specific corner of the genre, and honestly, navigating it can feel like a bit of a maze. You'll have the most luck diving into niche digital marketplaces—places like Smashwords are an absolute powerhouse for indie-published works with very precise tags. Authors there aren't constrained by the same content filters as the big retailers, so they can label things accurately. You want to search using the full string 'lesbian spanking hard romance' or break it down into 'FF spanking,' 'domestic discipline,' 'BDSM lesbian romance.' The 'hard' qualifier is tricky; it might mean emotionally intense, explicitly physical, or featuring power dynamics with real consequences. I'd recommend checking out authors like Tamsin Flowers or T.S. McKinney for FF BDSM, though their specific spanking titles might be MF. You'll often find that dedicated erotica writers will have a few FF titles in their back catalog even if they primarily write MF.
Don't overlook the potential on Amazon Kindle either, but you have to speak the language. The search algorithms are prude, so you're better off finding a book that hits two of your three keywords, then scrolling through the 'customers also bought' carousel. It's a treasure hunt. I found one called 'Her Rules' by an author named Miranda Silver that had some of those elements, though it was more psychological. Reader reviews are your best friend here—look for ones that explicitly mention the dynamic you want. If someone writes 'the punishment scenes were intense,' that's usually a good sign.
Also, communities on sites like Goodreads, despite the purge of certain adult content years ago, still have private groups dedicated to kink-friendly romance. You might have to request to join, but those are goldmines for recommendations from actual readers. People post 'ISO' threads all the time for tropes this specific. Just be prepared that a lot of the most hardcore stuff circulates as PDFs or on personal blogs rather than through official channels, which is a whole other ethical and practical can of worms.