My brain automatically goes to FanFiction.net for this one, just because that's where the massive archive lives. The sheer volume of Clemont x Serena fics there is kind of overwhelming, honestly. You can filter by pairing and just go wild, though the quality is a real mixed bag. Some are these incredible, thoughtful AUs, and others are... not that.
AO3 is where I find the stuff that feels more carefully crafted, especially for a ship that's kind of built on friendship and mutual support. The tagging system makes it easier to avoid the tropes I'm not into. I've also seen a surprising amount of activity on Wattpad, but the style there tends to be a lot more... dramatic, let's say. It's a different vibe, but sometimes that's exactly what hits the spot.