Chart found in https://www.cave-stg.com/forum/index.php?topic=281.0
Credit goes to EOJ

The items are found by either destroying enemies or the environment, you should not have much trouble finding them.
Note that they do not expand your health bar, they only serve to heal you. To get actual extends, you must reach the score thresholds (20 million and 45 million by default).

I've written it out for personal use, as I find it easier to visualize:

Chapter 2

Graveyard level 3 Half Life Up

Swamp level 2 Half Life Up
Swamp level 3 Bomb item

Volcano level 3 Half Life Up

Chapter 3

Graveyard level 3 Bomb item

Swamp level 2 Half Life Up
Swamp level 3 Bomb item

Volcano level 3 One Life Up

Death Mode (all level 3) only:
Double Extend Port Town level 3

Canyon

Bomb item

Hades Castle

Two Life Up (Defeat the first phase of Jitterbug without bombing)