I think you're confusing tokens and token cards.
Tokens are game objects that are generated by various spells and permanents. They are never part of your deck and indeed can not exist outside the battlefield. If a token moves to, for example, the graveyard, it vanishes instantly after moving, as far as the rules are concerned.
When an effect tells you to create tokens, you just do it—they exist now on the battlefield and don't “come from” anywhere inside the game. You can represent them with anything you want, even dice or scrap paper or a basic land with sharpie on it, but Wizards of the Coast also prints token cards you can use.
Token cards are cards that you can use to represent tokens. While they're by no means the only way to represent tokens (see above), they are by far the most convenient way to do so, as you can use the cards to easily represent various game effects, like their tapped state or whether or not they have counters on them.
Token cards don't go in your deck. You keep them with you while you're playing, maybe left inside your deck box. When an effect instructs you to create a token you can take one of those out and put it on the battlefield to represent the token.
You can find token cards from various booster packs, and pre-constructed products. They are not in any way tied to these products and can be used anywhere. Various sites also offer custom token cards with their own artwork. Some are two sided, so you can use the side you need (ie. the side that represents the token that was created). As far as the rules are concerned, the other side does not exist.