Where do you even find Eevee? I have a feeling that I missed it now.
Castelia Park. When you go into Castelia Sewers, go across the first bridge as usual and take the next bridge on your character's left and then a right. Before you fight the Plasma Grunts a guy is blocking the way but he will be gone after you defeat them. Eevees have a 5% chance of appearing like Riolus.