This is honestly one of those ships where the quality can be all over the map, and where you look depends heavily on your tolerance for different flavors of content. AO3 is the undisputed center of gravity for this pairing in English—it’s tagged as Madara/Tobirama or Madara/Uchiha Madara/Hatake Tobirama depending on the fic. The filtering there is a lifesaver. You can set the rating to ‘Explicit’ and maybe add ‘Dead Dove: Do Not Eat’ if you’re after the darker, more graphically mature stuff that isn’t just about smut but also intense power dynamics or historical animosity played out in... creative ways. That tag is crucial because some fics under the Mature rating are just plot-heavy with a single scene, while others are full-on explorations of their enmity-as-foreplay.
I’d warn against Tumblr for a focused search unless you already know specific blogs. The tag system is a mess, and you’ll mostly find fragments, moodboards, or short headcanons rather than complete longfics. Discord servers dedicated to Naruto rare pairs can be goldmines, though. You often need an invite from someone already in the community, but once you’re in, people share links to Google Docs or smaller archive sites that host content too explicit or niche even for AO3’s guidelines. There’s a whole ecosystem of ‘after dark’ rec lists on certain forums that get passed around. The trick is balancing the desire for mature themes with actual writing quality; filtering by kudos and bookmarks on AO3 usually surfaces the standouts that handle the complexity of their relationship well, not just the porn-without-plot ones.