Where does a lightning get its energy?

There are charges everywhere. A thundercloud experiences a lot of turbulence which separates charges inside the cloud (source 1). As a particular charge accumulates at the bottom of the thundercloud, the cloud and the ground act like two plates of a capacitor. The energy stored in a capacitor is proportional to the charge difference across the plates (source 2). It is also proportional to the area of the plate. Since clouds are massive objects, they have a lot of area.

The most crucial element, however, is air. Air is a very good insulator. This prevents charges from easily flowing to the ground. If charges can't easily flow to the ground, they can accumulate more. More charge accumulation = more energy being stored. Now eventually there is so much charge that the voltage (due to repulsion of like charges) forces free charges into the air.

These charges rip through the air, ionizing it. Ionized air is conductive, so this creates a path for more charges to flow. It's kind of like adding water behind a dam until it cracks from the pressure. Once it cracks, it gets weaker and allows more and more water to flow until it suddenly gives and all the stored water flows out in a huge devastating rush.

A majority of lightning bolts don't actually strike the ground. They just discharge between clouds. The ones that hit the ground are the most powerful because it takes a lot of voltage (and therefore more charge buildup) to break the huge air barrier.

It comes from the difference in potential between the clouds and the ground. It works the same as a capacitor. The charge (static electricity in this case) builds up on one side until the dielectric (air in this case) fails.

Ever been shocked by static electricity? Your body builds up a charge and when you reach for something like a door knob a little bolt of lighting shoots across when your hand gets close enough. Lightning is the exact same thing, but the scale is significantly larger. Think of the clouds as you, the ground as the door knob, the gap between the clouds and the ground is 1000s of feet. Takes a lot of energy to make that jump.

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lightning gets its energy from the motion of the air, water drops and ice crystals in the clouds.

Some of the kinetic energy of that motion is converted to electric potential energy as the particles collide and rub against each other.

This is somewhat analogous to rubbing a rubber rod with a piece of fur

where electrons are transferred from the fur to the rod. Some of the KE of the rubbing motion is converted to electric potential energy by the redistribution of electrons.

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As the air churns within the clouds, it carries electrons with it. And they become deposited at the bottom of the clouds over the entire area of the storm. And that area can cover whole sections of the country at a time. So we're talking LOTS of electrons.

As they gather at the bottom of the cloud formations, their E fields force the electrons near the surface of the planet to move away. Like charges repel. With a dearth of electrons in the ground, that causes it to be the anode. Note it's not that there are no electrons in the soil it's just that there are relatively fewer. And that make the ground (most of the time) the anode.

Suggest you browse "how many electrons gather in a storm cloud" on the web. You'll get lots of hits explaining the whole thing in detail.

Air is full of static electricity which develops due to friction between moving objects such as: the air and the ground, the air and the clouds, the air and water droplets in the air, air and air (turbulence), etc.

When there is enough of a charge to neutralize itself by arcing to the ground, you get lightning.

Actually, it perfectly explains how lighting becomes so powerful. There are literally hundreds of millions of volts building up over time and over a massive area. When it discharges, all that voltage is dumped in a fraction of a second through the air.

the energy comes from the fast moving electrons from the negative charge to the positive charge.. the kinetic energy of the electrons are lost as some heat energy as well.. and since electrons have a speed very close to that of speed of light, you can imagine how much heat they can generate :)