Every option has advantages and disadvantages.
1. Secondary market DDM singles. The price is pretty reasonable and there's a great selection. They can be representative for everything if you get a good selection but you don't need enough to have the PCs and important NPCs/villains look like what you want.
2. Fiery Dragon Creature counters etc. Advantage: cheap. You get a bunch at a low price. Disadvantage: they're two dimensional so poor lighting situations can make it hard for players to tell which is which (glare, etc). Also, they are thin and it's easier to move them accidentally. You also have to be organized with storage. If you just toss them all in a ziplock bag you'll never be able to find the one you want.
The papercutter at work or school and a flatened cereal box or other source of cardboard could also create these on the cheap if you felt like just numbering a bunch of paper chits.
3. "Dice minions" Go to your local gamestore and buy a bunch of d12s from the cheap dice section and a pack of round miniature bases. Superglue the dice to bases with numbers from 1-12 facing up. If you need more, pick up another color set of d12s.
They have the advantage that, as 3d representations, they look nicer on a battlemap than squares or two dimensional counters (especially if the PCs are represented by minis) and are easier to read. Also, because they are non-representative, you don't have the issue of the artwork being wrong for what you are representing. ("Everyone remember, the skeleton with the sheild and spear is the beautiful princess and the orc zombie with the axe is the halfling merchant, the woman wielding the axe is the succubus, and the three goblin archers are actually ghosts"). It can be a challenge to remember which is which but at least the appearance of your counter is not actively deceptive (which can happen if you proxy counters or minis that are unlike what you are proxying them for).
For large monsters, the 2 inch square Games Workshop monster bases make great proxies but dice or other markings would make them easier to tell apart.
4. Glass beads/ M&Ms/ candy.
Placeholders like this can be handy and are probably the cheapest option other than making your own counters. They are great when you need lots of minis for interchangable bad guys without enough hit points to take more than one hit. However, when you have to remember which M&M has 5hp of damage and which one is uninjured, it can lose its charm.