If you're hunting for Lilia Vanrouge/reader fics, I'd honestly start with Tumblr and then maybe pivot to AO3 if you're looking for something more substantial. There's a really specific charm to Tumblr's format for these—tons of quick, dialogue-heavy imagines and shorter fics that hit the spot when you just want a quick dose of that dynamic. I've seen some great ones tagged #lilia vanrouge x reader or #lilia vanrouge imagine.
AO3 is where the real deep cuts live, though. Filtering by the 'Lilia Vanrouge' character tag and then sorting by kudos or bookmarks usually surfaces some longer, more plot-driven pieces. The tagging system there is a lifesaver; you can often find stuff under 'Fluff' or 'Hurt/Comfort' depending on your mood. I'd avoid Wattpad for this specific pairing—the signal-to-noise ratio felt off last time I looked.
My personal favorite find was a college AU series where Reader is a struggling magic history student and Lilia is the annoyingly competent, centuries-old TA. It shouldn't work, but it does.