You either have to get it from the Old Lady from the Battle Shop in Emerald to teach it to a Chansey on there, which costs 24 BP (Battle Points), or go to the Move Tutor in the Pewter Museum from Fire Red or Leaf Green and get the tutor to teach it to a Chansey. I think that is how you do it.
how do i get a chancy with sesmic toss[/QUOTE]
Fr/Lg, (Fire Red or Leafgreen. As a Move Tutor Move.