I can cover what is legal and what is common, and you can hopefully determine what is "convenient".
The MTG Tournament Rules dictate that you clearly represent your board state.
This means: You must be able to represent the tapped state of
your token You must be able to differentiate types of tokens Tokens must not be in the same sleeves as the deck you are playing
This is under Section 3.8 of the Tournament Policy [...]
Sleeves or card backs that appear similar to any player’s sleeves or card backs may not be used as markers.
[...] Notably, if you are playing without sleeves, you may not use an unsleeved Magic card as a token
I see no reason to ignore these rules for casual play.
They aren't very limiting.
The most common legal method of representing a token, for those lacking the actual token card, is a plain face down card.
Any card.
If you control multiple types of tokens, it must be clear to both players which cards represent which tokens.
Some players use dice to represent tokens.
Depending on what kind of dice you use, it can be difficult or impossible to tell whether a particular token is tapped or untapped.
It's also unclear whether a "2" means "two tokens" or "one 2/2 token".
You can overcome this by using dice that don't have rotational symmetry and by communicating clearly.
I still advise against it because there are easier, clearer options (my opinion).
Representing a large number of tokens individually may not be feasible.
In these cases, you can group tokens that have the same status using a single card with a numeric counter placed on top.
Every token in the group must be identical.
If some are tapped, or if some have summoning sickness, represent them as a separate group.
boardgames.stackexchange.com/users/6692/Rainbolt
When ever I play magic with my brother or friends, we use sticky notes.
You can write any abilities they might have just as long as you bring a pen.
I use smaller sticky notes as +
+ tokens too.
This method is occasionally used for feature matches (i.e., matches that get video coverage) at large tournaments, but only when a better alternative is not available.
For example, there is no official marker for which mode is chosen for Outpost Siege (Dragons or Khans), so a sticky is used instead.
boardgames.stackexchange.com/users/13342/Rudemeany
Most counters and methods use dice or cards to represent that there is a token, or that it has counters on it.
But there is only one product I've found that displays power/toughness and can double as a tappable creature token on its own.
It's from Jace's Toy Chest.
Only place i've found them is on Etsy and ebay.
boardgames.stackexchange.com/users/17518/Spartacus33
My friends and I use cards that aren't in the current game, face-down.
We put coins (1, 2 or 5 cents) on them depending on the power/toughness of the creature tokens (since most of the time, they have equal power and toughness).
boardgames.stackexchange.com/users/8542/ThomasP
Our group uses index cards, which is pretty handy because you can just write the static stats (color, creature type, BASE P/T, etc) on the card.
D6 on top of the card is usually enough for counters, and d10/20 to count a group of identical tokens if things start getting... silly...
boardgames.stackexchange.com/users/13240/user13240
I put all my ad cards in white, blue, green, red, or black sleeves and put those face down with a dice on them.
boardgames.stackexchange.com/users/12630/andrew
Mini Dice and flipped cards.
My friends typically up just use dice.
If you want you can easily get a lot of token cards since almost all of them are super cheap.
boardgames.stackexchange.com/users/14223/Christian Muller
As far as dice go, use two dice for a 2/2 token.
When they are horizontal, the token is untapped.
Twist them both so they are vertical, and it's tapped.
boardgames.stackexchange.com/users/14244/Kaspar
I just use a face-down card and put a piece of paper on it with its stats
boardgames.stackexchange.com/users/21208/Red Dawn
Many spells or abilities cause one or more creatures to join the battlefield (i.e. 1/1 white Soldier creature tokens, 3/3 green Beast creature tokens, etc.). These token cards are not included with the cards that require them.  What do MtG players use instead to represent token creatures?  A quick search shows you can buy packs of such cards, but I'm wondering what convenient ways exist to represent these tokens without actually having to buy more cards?
What should be used to represent creature tokens when the printed token card isn't available?
boardgames.stackexchange.com
<magic-the-gathering>
boardgames.stackexchange.com/questions/20302
boardgames.stackexchange.com/users/2260/Joe Golton
The most common method of representing a token is to leave it face down. You can also use dice or index cards. The most common legal method of representing a token is a plain face down card. You can put a piece of paper on it with its stats. Some players use dice to represent tokens. Index cards are useful because you can write the static stats on the card. Sticky notes are another useful tool.
The most common legal method of representing a token is a plain face down card. You can put a piece of paper on it with its stats. Some players use dice to represent tokens. Index cards are useful because you can write the static stats on the card. Sticky notes are another useful tool.