Cheapest priced copy (Dragons of Tarkir): $0.49 at Card Kingdom, near mint, their price list as of .
Gleam of Authority 
Enchantment — Aura · Rare · Dragons of Tarkir · #19
Enchant creature
Enchanted creature gets +1/+1 for each +1/+1 counter on other creatures you control.
Enchanted creature has vigilance and "{W}, {T}: Bolster 1." (To bolster 1, choose a creature with the least toughness among creatures you control and put a +1/+1 counter on it.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 19 | Rare | $0.49 | $0.79 |
2 rulings from Wizards of the Coast
- 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.
- 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.