Honestly? I spent a ridiculous amount of time last year trying to track down good 'Shion x Narato' fics, and it was way harder than I expected. The pairing isn't super mainstream, so you're not gonna find a dedicated category on the big sites. FF.net has a few, but you really have to dig—search for 'Naruto & Shion' and then sift through the gen fics to find the ones with romantic subtext. The tags are a mess. A lot of writers just throw it under 'Naruto' or 'Naruto Various Pairings,' which is no help at all. I found maybe three decent multi-chapter stories there over six months.
Your absolute best bet is Archive of Our Own. AO3's tagging system saves lives for niche ships like this. You can filter by 'Naruto/Shion' as the relationship, and because people tag their tropes, you can find stuff like 'Post-Fourth Shinobi War' or 'Divergence from Canon' which often leads to more thoughtful character explorations. There's a writer called Kaze no Shion who does almost exclusively this pairing, and their stuff is pretty much the backbone of the tag. The quality on AO3 tends to be higher overall, but the quantity is still small—maybe a couple dozen solid fics total, which feels tiny compared to something like NaruSasu.
Don't sleep on some smaller forums either, though they're like digital archaeology. I stumbled on a treasure trove of older, completed stories on a now-dead Naruto forum that someone had archived on a personal blog. The writing was... of its time, let's say, but charming in a nostalgic way. Wattpad has some, but it's overwhelmingly dominated by self-inserts and the prose can be rough. It's a ship that survives on dedicated fans rather than a massive audience, so the good stuff feels like a real find.