Across the river (in the River) from where Vileplume is (or you may see it before that) there will be a Slowpoke on the grass. Like when you get Pikachu to move in the Tunnel to free Zapdos, you need to throw apples to lure Slowpoke near the Shellder sign. It will turn around. and dip its tail in the water. When it lifts the tail up get your camera ready because when Shellder hits Slowpoke's tail, Slowpoke will evolve into Slowbro.