Archived of Our Own, AO3, is basically the undisputed capital for this ship in my opinion. The tagging system is a lifesaver, letting you filter for exactly the kind of story you want, whether it's post-Tower of Nero fix-its, canon-divergent adventures, or pure fluff. You can track down fics that explore the Underworld diplomatic angle or just them being dumb teens at camp. The quality varies wildly, of course, but the sheer volume and the ability to find hidden gems through kudos and bookmarks makes it indispensable.
That said, I've stumbled upon some truly exceptional Solangelo fics on Tumblr over the years, often from writers who cross-post to AO3. The platform's strength is in the threads—those multi-part stories told in reblog chains, accompanied by moodboards and fanart. It feels more communal, like you're discovering a story alongside other fans, adding your reactions in the notes. The downside is discoverability; it's a nightmare unless you're already following the right blogs or get lucky with a reblog.
Fanfiction.net still has some classic, longer-form stories from the late 2000s and early 2010s, a real time capsule of the fandom's earlier style. Some are surprisingly well-plotted. But honestly, the interface is so clunky and the lack of robust tagging makes it a chore to sift through. I only go back for a few specific authors I loved back in the day.