SplinterGlossary: Reward Card
They can also be purchased from the secondary market.
Unlike cards from booster chests, which have a chance of being revealed when opening a pack that depends on their rarity and boosts (for legendary and gold foil cards), reward cards have a print limit for each card.
Before the print limit is reached, they still have a chance to be discovered in loot chests. Once reached, no new copies of that reward card will be issued.
When one or more reward cards run out of print, they will soon be replaced with another.
The print limit for every reward card can be checked from this website.