Finding good 'Powerpuff Girls' and 'Rowdyruff Boys' crossovers is like digging through a mixed bag of glitter and old crayons. You get some really shiny stuff, but you have to sift through a lot of crumbled bits. My absolute favorite hub is Archive of Our Own because the tagging system is a lifesaver. You can filter by pairing, like if you're specifically looking for Buttercup x Butch dynamics or exploring the more chaotic possibilities with the whole group. The 'crossovers & fandom fusions' tag combined with the character tags will get you right there.
A lot of the older, classic stuff tends to be on FanFiction.net, though. The search function is a nightmare, so I'd recommend Googling something like "PPG RRB crossover site:fanfiction.net". That usually surfaces the fics that have stood the test of time. The writing quality can be a real toss-up, but some of those early 2000s fics have a certain charm and wild creativity you don't see as much anymore.
Honestly, I've had better luck with smaller, dedicated Discord servers these days. The fandom isn't as huge as it once was, so the passionate writers and readers have sort of clustered in little communities. You find links to Google Docs and forum threads that never got posted to the big archives. It feels more like trading secret notes, which is kind of fun if you're looking for something off the beaten path.