Sylveon locations — where to catch

In Lumiose City (Legends: Z-A)

• Wild Zone 20 — Walk

In Paldea (Scarlet & Violet)

• Dalizapa Passage — Walk
• North Province (Area Three) — Static Encounter
• South Province (Area Six) — Walk
• West Province (Area Three) — Walk

In Hisui (Legends: Arceus)

• Space-time distortions — Walk

Other games (2)

• Sword / Shield — The Isle of Armor (Gen 8): Ballimere Lake, Giant's Bed
• Sword / Shield (Gen 8): Lake of Outrage

Sylveon — full Pokédex entry