I was considering finding a Yuki-Onna and a Decanter of Endless Water and letting nature take its course.
Edit: Teleport/Greater Teleport to a frozen climate and/or Create Demiplane for a cold environment could also work.
The Cost of Undeath
This section is a work in progress, sorry it's sparse.
Being able to animate the dead is only half the battle, the other half is acquiring corpses for the task.
Shovel, common item (This tool lets you dig a pit at a rate of 2 cubic feet per minute.)
Undead are expensive, not as expensive as Constructs but an aspiring necromancer will often find it necessary to replenish their horde. This is where reducing, or even negating, the cost of the requisite onyx can come in handy.
Blood Money spell
Component Freedom archmage mythic path ability
Craft (gemcutting) skill (Or the Alternate Crafting Rules might be able to reduce the cost of the onyx if you "craft" them yourself.)
False Focus feat
Mythic Animate Dead ("By expending a second use of mythic power, you can ignore the spellís material component cost.")
Mythic Eschew Materials feat
Symbol of the Holy hierophant mythic path ability
Create Soul Gem + Soul-Powered Magic Will let you convert slain creature's souls directly into material components for spells.
Yuelral's Blessing: You may replace the material component of any arcane spell with gems of the same value.
Animating undead can often incur other costs, the copious volumes of ice required for Frostfallen creation for example.
Mythic Animate Dead ("...ignore the spell's material cost." could arguably allow one to ignore the ice and additional gems creating Frostfallen requires.)
Spell Component Pouch item (ice is cheap, perhaps cheap enough that a spell component pouch could hold a nigh infinite amount of it? Ask your GM.)