You can purchase Rank 2 from the Stolen Royal Vendorbot in Rustbolt. It requires the following to purchase:

-Revered Reputation with Rustbolt Resistance
-S.P.A.R.E. Crate x 20
-Galvanic Oscillator x 10

Rank 3: Откалиброванная шкала бытия

Edit: It looks like if you purchase one of the other Rank 2 Essences at the same vendor, the rest of your unpurchased Rank 2 Essences get a discounted price. You have to learn it to your Heart Forge first though to see the discount on the other Essences.

Комментарий от Aisenfaire

The Well of Existence

Major Power: Восполнение сил (Instant, 15 sec cooldown)

Rank 1: Release all healing stored in The Well of Existence into an ally. This healing is amplified by 20%.
Rank 2: Release all healing stored in The Well of Existence into an ally. This healing is amplified by 50%.
Rank 3: Release all healing stored in The Well of Existence into an ally. This healing is amplified by 50%. The Well will not release more healing than needed to fully heal the target.

Minor Power: Источник бытия

Rank 1: 15% of your overhealing is stored in The Well of Existence. When you heal a target under 40% health, the Well releases up to 5184 of its stored healing into them.
Rank 2: 15% of your overhealing is stored in The Well of Existence. When you heal a target under 50% health, the Well releases up to 5184 of its stored healing into them.
Rank 3: 15% of your overhealing is stored in The Well of Existence. When you heal a target under 50% health, the Well releases up to 5184 of its stored healing into them. When the Well contains less than 13,252 healing, it absorbs healing twice as quickly.

Drop Location: Mechagon Reputation

Rank 1: Obtain Rustbolt Resistance reputation Honored and buy it from the Quartermaster for 2 Ящик "ЗПЧСТ" and 2 Гальванический осциллятор. Cost reduced to 2 Ящик "ЗПЧСТ" and 1 Гальванический осциллятор after learning Rank 1 of one Mechagon Essence.
Rank 2: Obtain Rustbolt Resistance reputation Revered and buy it from the Quartermaster for 20 Ящик "ЗПЧСТ" and 10 Гальванический осциллятор. Cost reduced to 8 Ящик "ЗПЧСТ" and 4 Гальванический осциллятор after learning Rank 2 of one Mechagon Essence.
Rank 3: Obtain Rustbolt Resistance reputation Exalted and buy it from the Quartermaster for 40 Ящик "ЗПЧСТ" and 25 Гальванический осциллятор.
Rank 4: Obtained from Ржавоболтские припасы.

Items:

Source: https://www.wowhead.com/guides/heart-of-azeroth-essence-overview

Note: Rank 4 is cosmetic only, which is why the figures provided only matter up to rank 3.

July 12 Hotfix:

The cost of the Rank 3 Essences available from the Rustbolt Quartermaster has been reduced to 15 Galvanic Oscillators (was 25 Galvanic Oscillators) for the first Essence. Subsequent Rank 3 Essences now cost 6 Galvanic Oscillators (was 15 Galvanic Oscillators).

Source: https://news.blizzard.com/en-us/world-of-warcraft/23036677/hotfixes-july-16-2019

Комментарий от tkalfaoglu

It seems the SPARE crates are no longer bought but made. In fact, neither item for Rank 2 is sold by the quartermaster..
But they can be made by the production robot there.

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Усиленный конденсатор энергии бытия

2-й уровень
Уровень предмета: 50
Становится персональным при получении
Уникальный

Использование: Наполняет Сердце Азерот силой "Источника бытия".
Ржавоболтское сопротивление должно уважать вас. (Покупка невозможна, если талант уже открыт.)

Мощная способность

Восполнение сил

Радиус действия: 100 м

Мгновенное действие	Восстановление: 15 сек

Передает сохраненное в Источнике бытия здоровье союзнику. Это исцеление усиливается на 50%.

Малая способность

Источник бытия

15% вашего избыточного исцеления сохраняется в Источнике бытия. Когда вы исцеляете цель с уровнем здоровья ниже 50%, Источник передает ей до 1371 ед. сохраненного здоровья.

Усиленный конденсатор энергии бытия

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