I'm guessing you want the fics that are more on the fluffy, comedic side of the 'ShiroGiku' dynamic, right? Where he's exasperated but fond, and she's just chaotic. AO3 is absolutely the main hub now, hands down. The tagging system is your best friend here - you can filter for the pairing 'Kurosaki Ichigo/?' wait no, that's wrong... it's under 'Hitsugaya Toushirou/Matsumoto Rangiku', and then add tags like 'Fluff', 'Humor', 'Crack', or 'Lighthearted'. I find the ones that are just pure, uncomplicated fun usually have 'No Angst' or 'Fluff and Humor' in the summary. There's a classic from years back, 'Snow and Cherry Blossoms', that gets recced a lot for this vibe - it's all about Rangiku trying to teach him to relax and him being hilariously bad at it.
I used to read a ton on FF.net too, but the search there is such a pain. You kinda have to dig through the muck of older, more angsty fics to find the lighthearted ones. The good news is, once you find an author you like on either site, you can usually binge their whole catalogue. I've noticed the really funny, character-focused ones tend to get a lot of comments from people just thanking the author for the serotonin boost, which is always a good sign.