Experience Cards are cards exclusively used for fusing with Servants in order to increase their levels. These cards each have a respective Class and number of Stars. More Servant EXP is gained the higher the number of Stars and if the Experience Card has the same Class as the Servant.
Experience Cards from 1 to 3 stars can be acquired through the Friend Point Gacha, and all of them from Daily Event Quests in Chaldea Gate: specifically, Monday, Tuesday, Thursday, and Friday.
Starting of each new month, you can buy a 10x 4★ All-Classes Experience Card for 40 Mana Prisms for each Bundle on the Da Vinci's Workshop Mana Exchange section.