You'll need a male Pokemon with Leech Seed that can breed with Snover or Abomasnow. This list contains: Bulbasaur line, Exeggcute line, Hoppip line, Sunkern line, Shroomish line, Roselia, Roserade, Cacnea line, Turtwig line, or Cherubi line.
Once you have one of those Pokemon as a male with Leech Seed, get a female Abomasnow or Snover, and put them in the Daycare together. Check back every so often to see if an Egg was produced.
Once you receive an Egg, walk around until it hatches into a Snover. This Snover should have Leech Seed.
Source: Experience, this
Hope I helped!