You use breeding.  Breed a male Wooper/quagsire, Dunsparce, Slacoth/Vigoroth/Slaking, Pansear/Simisear, Bidoof/Bibarel, Hippopotas/Hoppodwen, or Munna/Mushanara who knows yawn (these are compatible pokemon who learn it through leveling up.  Smeargle is also a candidate) with a female Eevee and the baby Eevee will know it.  Eevee/Espeon cannot learn grass whistle nor sweet kiss.  Also, sweet kiss doesn't induces sleep, it causes confusion.