Every time someone asks about where to find Anko x Naruto stuff, I have to laugh a little. Those two have such a specific, weirdly appealing dynamic, but it’s never gonna be front-page material on the big sites. You have to dig. FanFiction.net still has the biggest archive, hands down. Use the filters: sort by reviews or favorites, not follows, and set it to 'Naruto' fandom, pairing 'Naruto U. & Anko Mitarashi'. You'll get a mix, but the classics from like 2008–2012 are there.
Archive of Our Own is trickier because the tagging for them is all over the place. Sometimes it's under 'Anko Mitarashi/Naruto Uzumaki', other times it's just a side tag in a harem fic. Searching by kudos works, but honestly, I've found better stuff through curated Tumblr rec lists from ages ago. People used to be obsessed with 'redeemed Anko mentors Naruto' plots, and some of those are genuinely well-written.
Word of warning: a lot of what you'll find leans heavily into the... adult side of things. If that's not your jam, filtering out the 'M' rated stuff on FFN or using AO3's exclude filters is a must. The real top-tier ones, in my opinion, are the slow-burn missing-nin adventures where they both end up on the run. There's one called 'Snake's Vengeance' that did the rounds on forums, but good luck finding a clean download link now.