Really? i found .this page in less than 5 minutes, google before you ask questions like this please.
http://lmgtfy.com/?q=where to find false swipe in soulsilver?
You can find it in the Dark Cave or buy it at the Goldenrod Department Store for 2000 pokedollars
Hope i helped, if i did you can help me by clicking the golden star in the top corner :D
In Platinum you can get false swipe at the Veilstone Department store.Hope i helped:)