Charms can only be obtained from the Main Shop, right below the unit banners. You need to pull military cases and each pull gives a charm of random rarity (from Common to Legendary).
The currency for opening charm crates is calling Red Flares.
Where to find Red Flares?
Red Flares used only for opening charm crates. You can get it:
As a reward for completing the Team Power Collection.
When reaching rank 6.
Can be purchased in the Shards Shop.
Or obtain for green money.
All your charms are stored in a separate charms tab in your inventory.