Stephanie Plum fanfiction is surprisingly vibrant, and I love diving into it whenever I need a fix between Janet Evanovich's books! Archive of Our Own (AO3) is my top recommendation—it's got a dedicated tag for the series, and the quality ranges from hilarious one-shots to multi-chapter mysteries that feel like lost Plum adventures. Some writers nail the Trenton vibe and Stephanie's chaotic charm so well, I forget they're not canon. FanFiction.net has an older but solid collection too, though you might need to sift through abandoned fics. Tumblr occasionally hides gems in character-focused blogs, especially for Ranger/Morelli dynamics.
For deeper cuts, check out smaller forums like FanLib or Wattpad (search tags like #BountyHunterRomance). The fandom isn't as massive as, say, 'Harry Potter', but the stories often mirror Evanovich's blend of action and humor. I stumbled on a noir-style AU last year where Stephanie teams up with Joe from 'Die Hard'—absolutely unhinged but genius. Pro tip: Sort by kudos/comments on AO3 to find hidden masterpieces. Those late-night rabbit holes are how I discovered a writer who exclusively plots Lula-centric heist stories.