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The Shadow of Moil spell requires an undead eyeball inside a gem.

Who would be selling such an item and if someone was selling it, how would they have crafted it?

As a side note Wish and Divine Intervention are not valid answers as they are not certain to work in this right.

An undead eyeball

Acquiring an undead eyeball itself seems relatively trivial for the unethical necromancer; simply kill someone, animate them as a zombie using Animate Dead, and then pluck an eyeball from your new zombie friend.

I have interpreted this requirement to mean the eyeball of an undead creature, not specifically an eyeball that is an animate undead creature in its own right; but it's a good staple of undead tropery that the severed parts of a zombie remain independently animate, and as a DM I would... happily?.. let you acquire an animated undead eyeball via this procedure anyway.

Encased in a gem

You should be able to manage this part of the component by use of one large, expensive gem (not more than 1ft in diameter), a set of jeweller's tools, and the Mending cantrip. Simply split the gem stone with one clean break; carve out a hollow space inside the gemstone large enough to contain your undead eyeball; put the eyeball within, press the halves of your gemstone back together, and use Mending to seal the break in the gem. Mending can only fix a single break or tear at a time, so you should be able to seal the original break without affecting the carved hollow. Voilà; an undead eyeball encased in a gemstone.

The services of someone proficient with jeweller's tools might be advised for this process, since making a clean break is probably tricky for someone just waving a hammer and chisel around without knowing what they're doing.

You pay 150 gp. As it is a material component with a stated cost, in order to acquire the item, you simply need to buy one by paying the price listed in the book. Where did it come from? Doesn't really matter, according to the game mechanics - the exact nature of the material component is largely just fluff.

Now, from a more in-universe standpoint: You make pearls. Pearls are a form of gem, and they're created by clams forming layers of calcium carbonate around some form of irritating matter within their shell. So, you cast Create Undead on a Small humanoid, surgically remove the eyeball and preserve it to prevent decay, then place it inside the shell of a sufficiently large clam and wait for the natural processes to coat it with a pearl.

The only official option for players is imprisonment

There is only one method described in the rules to encase something in a gemstone and that is the Minimus Containment option of the imprisonment spell:

The target shrinks to a height of 1 inch and is imprisoned inside a gemstone or similar object.

Now imprisonment can only target a creature, but if you true polymorph an undead eyeball (preferably a particularly large one, see below) into a creature, imprison it, and then end your true polymorph, you can get an eyeball in the gem.

You can turn an object into any kind of creature, as long as the creature's size is no larger than the object's size and the creature's challenge rating is 9 or lower.

The game world is not limited in this way

All that being said, it is doubtful that the only way to acquire the components for a level 4 spell is via two level 9 spells. As such, it should be assumed that it is possible for artisans of some description to accomplish this.

The written rules for player characters don't include everything available in a game world. It is possible that the game world has other mechanisms by which such a piece could be crafted, and either your PCs, some NPCs, or the world itself can develop these reagents. You'll have to ask your GM for what other options may exist in your game.