http://www.smogon.com/dp/pokemon/chansey/moves
http://www.smogon.com/dp/pokemon/blissey/moves
chansey and blissey lack seismic toss in their movepools
http://www.smogon.com/dp/moves/seismic_toss
a lot of Pokemon, in general, are missing from the Seismic Toss page.. I can only remember the Regis atm, but I'm pretty sure there's more.
I don't actually know where they get it, but they're in their sets...