The Westchester man who killed his two children with a shotgun in a shocking murder-suicide rampage had bought the weapon more than five months earlier.

Sam Friedlander, who also killed his wife, bought the 12-gauge Remington 870 and two boxes of buckshot ammo on April 29 at a Yonkers sporting-goods store, State Police Maj. Michael Kopy said yesterday.

The disclosure indicated Friedlander had planned the murders early on in his bitter divorce from his wife, Amy.