This is actually a pretty niche ship in the 'Seven Deadly Sins' fandom, so the pickings can be slim depending on what flavor you're after. If you want the highest concentration and the most dedicated authors, Archive of Our Own is the undisputed champion for Meliodas x Zeldris. The tagging system is a lifesaver—you can filter for the 'Meliodas/Zeldris' relationship tag and immediately find hundreds of works, everything from short, angsty one-shots to epic multi-chapter fix-it fics.
What really sets AO3 apart for this pairing is the willingness to explore darker, more complex dynamics. You'll find a lot of stories that seriously grapple with their history, the demon clan politics, and that messed-up brotherly love-hate thing they have going on. It's not all sunshine, which honestly fits them better. Wattpad has some stuff too, but it tends to skew younger and more towards modern AUs, which isn't my personal jam for these two. For me, the best stories that actually make their canon baggage part of the tension are almost exclusively on AO3.