Ugh, trying to find a good Ichigo/Urahara fic is its own kind of hell. They're a surprisingly rarepair compared to the big ships, so you're not gonna find a dedicated hub. Your main hunting grounds are gonna be AO3 and FF.net, obviously, but the quality and volume are wildly different between them.
On AO3, the tag is pretty small—maybe a couple hundred works total? But the quality ceiling is way higher because writers there tend to focus more on complex, often mature or speculative character studies. You'll find the weird, introspective fics, the slow-burn political machinations where Kisuke's manipulations are a form of courtship, and the post-war 'what are we now' angst. The tagging system lets you filter for exactly the dynamic you want, which is a lifesaver. Just be ready to wade through a lot of 'background Ichigo/Urahara' or fics where they're just a side pairing in someone else's epic.
FF.net has more fics numerically, but it's a total gamble. You have to dig through mountains of poorly formatted, OOC crackfics and harem stories from the mid-2000s where Ichigo collects captains like Pokémon. Every once in a blue moon, you'll stumble on an absolute classic from that era that's been preserved, but it's a real archive dive. Tumblr used to have a tighter-knit community for this ship, with headcanon threads and snippet exchanges, but that's mostly migrated to Discord servers now, which are harder to find unless you're already in the circle. Honestly, most of my favorite ones for this pair are from dead LiveJournal communities I found via wayback machine links on TV Tropes.