Close to home. Use Surf at route 1, go to the island and head west. You'll find route 17 and 18 past the gate.
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Route 18
You can't, It is only obtainable in Pokemon Black or White. But if you want one in Black or White, a Pokemon ranger will give you an egg of a Larvesta in Route 18 so remember to make room for it!
Larvesta
route 18
go 2 route 18 and surf. you'll find a ledge. hm05 (waterfall) will be on the ledge.