Finding the good stuff for that specific niche takes patience, because you're really hitting two uncommon tags at once—the fem bijuu angle and then making it a harem. The crossover sections on big sites are my first stop, but honestly, a lot of the gems come from writers who don't even tag it perfectly.
Archive of Our Own is the powerhouse these days. Its tagging system is a blessing and a curse. You need to use the 'Naruto' fandom tag, then filter by 'Harem,' and maybe try 'Bijuu' or 'Jinchūriki' as an additional tag. The 'Female Bijuu' tag exists but is sparsely used; sometimes writers just put 'Female Kurama' or 'Matatabi' in the character relationships. I've found a couple by sorting by kudos after a broad filter.
FanFiction.net feels older, but there's a deep archive. The search is terrible, so you go into the Naruto category, set to 'English,' sort by follows, and then just...scroll. It's a grind. You'll see a lot of 'Naruto has a harem' summaries, and you need to open each one to see if any bijuu are part of it and if they're written as female. It's inefficient, but I've stumbled on some classics that way.
Don't sleep on smaller forums or dedicated Naruto fanfiction archives like 'The Fanfiction Forum' or 'NarutoFanfiction.com.' They have a more community-driven feel, and writers there sometimes explore very specific concepts. You might need to ask directly in a recommendation thread.
The quality varies wildly. Some stories treat the bijuu as shallow power-ups with a female form attached, which can be disappointing. The ones that feel high-quality usually delve into the ancient, alien perspective of a tailed beast suddenly experiencing humanity and relationships. They tend to focus more on character dynamics than just the harem checklist.