The real central hub for fanfiction is always Archive of Our Own. For something like Ban and Elaine from 'Nanatsu no Taizai', the search filters on AO3 are perfect because you can specify the fandom and then the relationship tag, usually listed as 'Ban (Nanatsu no Taizai)/Elaine (Nanatsu no Taizai)'. You can sort by kudos, bookmarks, or comments to get the most popular ones straight away. I find the tagging system there is really thorough, so stories will be marked with common tropes for the pairing, like 'Post-Canon', 'Fix-It', or 'Angst with a Happy Ending'.
If you're looking for quantity over curated quality, FanFiction.net still has a massive backlog. The Seven Deadly Sins category is pretty active, and you can filter by character. The challenge is that the sorting isn't as refined, so you might have to sift through a few pages of older stories to find the really stand-out ones. Wattpad can be hit or miss, but I've stumbled upon some surprisingly heartfelt Ban/Elaine AUs there—sometimes they're reincarnation or high school settings that really explore their dynamic from a new angle.
Honestly, don't overlook Tumblr. A lot of writers post shorter drabbles or headcanon threads there, and if you find a good blog dedicated to the ship, they'll often re-link to their longer works on AO3. The community feel is different; it's more about sharing snippets and reactions in real time. My last deep dive had me reading this one multi-chapter about them trying to navigate a modern domestic life, and it was strangely comforting.