Cheapest priced copy (Dragons of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Sandcrafter Mage 
Creature — Human Wizard · 2/2 · Common · Dragons of Tarkir · #33
When this creature enters, bolster 1. (Choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 33 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 25 February 2015Bolster itself doesn’t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Aven Tactician’s bolster ability.
- 25 February 2015You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves. That could be the creature with the bolster ability, if it’s still under your control and has the least toughness.