Where to Obtain Space Cards
Space Cards have very limited sources of acquisition compared to other Revelation Cards.Since the stats are also random, it's important to secure as many as possible to increase your chances of getting good stats.
Repeatable Sources

Key Source - Nightmare's Gateway (resets per version)

Key Source - Companio Shop Exchange

Premium Phantom Pass

Land of Fortune Exchange (prioritize obtaining and enhancing Wonder Weapons)

Rerun Contract Currency Exchange

Kagura Bell
Other sources vary by server, but they are included in paid packages.
One-time Sources

Limited TASK (1 time)

Baseball Point Exchange

Metro of Desire Level Rewards

Metro of Desire Achievement Rewards

Tycoon Tier Rewards

Tartarus (Persona 3 Collab) Achievement Rewards
Other than these, they can be obtained from various events.