Also, if you ever really run out of certain components:
• An investigator can gain a card or token only if it is available.
► A card is available if it is in the deck, discard pile, or reserve.
► Cards and tokens on a defeated investigator’s sheet are not available.
• When a deck of cards is empty, immediately shuffle its discard pile
to form a new deck. The Mythos deck is never replenished.
(Eldritch Horror (Base Game) Reference Guide, Page 3: "Component Limitations".)
Thankfully, the expansions provide us with additional variants of existing components.