Finding that specific pairing can feel like navigating a dusty corner of the shinobi archives. Most platforms that host more explicit fanfiction will have it, but you often have to dig. Sites like Archive of Our Own and fanfiction.net are obvious starting points, but you'll need to master their search filters and be prepared to sift. The pairing itself isn't in the top tier for 'Naruto', so stories are fewer and often mixed in with broader team-based collections or older fics from when the series was airing.
I've had more luck with smaller, forum-based communities dedicated to 'Naruto' pairings, though they can be hit or miss and sometimes vanish. The challenge is that Kurenai is a side character, so writers exploring her dynamic with Naruto often build it into longer, plot-heavy fics, and the explicit content might be just a chapter or two rather than the entire focus. It's a niche within a niche, honestly.
Last time I went looking, I think I found a couple by searching the tag 'Naruto/Yuuhi Kurenai' on AO3 and sorting by a combination of kudos and completion date, but that was ages ago. Good luck, hope the hunt turns something up.