From The Wiki:
"Gold Finger is an item pickup that provides a 14 seconds period of invulnerability, and 10 seconds of set enemies spawned to 500 and -100% Cooldown, and offers variable rewards after said period ends. It is the rarest drop from destroyed light sources."
The First Place You Can Find Them Is On Astral Stair.
- Afterwards, You Can Find Them On Any Map.
Collecting 5 Total Unlocks The "Space 54" Map, Which You Need For 5-6 Other Achievements.
Good Luck!