Finding those fics can be a real adventure! Honestly, I'd head straight to Archive of Our Own and filter by the Felix/Ferdinand von Aegir & Oswald tag. The volume there is just unmatched, and the tagging system lets you zero in on anything specific you're craving, whether it's enemies-to-lovers or post-canon fix-its. Sometimes I get overwhelmed by the sheer number, so I sort by kudos to find the real crowd-pleasers first.
A lot of the really classic, longer works from the fandom's peak activity are still up on FanFiction.net, though the tagging isn't as precise. You have to dig a bit more manually, which has its own charm—I've stumbled on some absolute gems I'd never have found otherwise by just scrolling through the 'Fire Emblem: Three Houses' section. The prose in some of those older ones feels different, more formal maybe, but I've got a real soft spot for them.
Don't sleep on Tumblr either. A lot of writers will post shorter snippets, headcanons, or even link to their AO3 works directly from their blogs. The interaction feels more immediate, and you can sometimes find WIPs or ideas that haven't made it to the big archives yet. I just really love that sense of a living, breathing community sharing little bits of the ship every day.