Cheapest priced copy (Ultimate Masters): $0.79 at Card Kingdom, near mint, their price list as of .
Sovereigns of Lost Alara 
Creature — Spirit · 4/5 · Rare · Alara Reborn · #12
Exalted (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.)
Whenever a creature you control attacks alone, you may search your library for an Aura card that could enchant that creature, put it onto the battlefield attached to that creature, then shuffle.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.79 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Alara Reborn | 12 | Rare | $0.99 | $5.99Out of stock |
| Ultimate Masters | 207 | Rare | $0.79 | $2.99 |
7 rulings from Wizards of the Coast
- 1 May 2009If the attacking creature has left the battlefield by the time Sovereigns of Lost Alara's second ability resolves, you can't put an Aura onto the battlefield this way. You may still choose to search your library just to shuffle it.
- 1 May 2009Any Aura card you find with Sovereigns of Lost Alara's second ability must be able to enchant the attacking creature as it currently exists. You need to check the Aura's enchant ability as well as any effects, such as protection, that would make it illegal to attach that Aura to the attacking creature. For example, if the attacking creature were an artifact creature with protection from blue, you could find an Aura with "enchant artifact," but you could not find a blue Aura.
- 1 May 2009The attacking creature is not targeted by either this ability or the Aura, so a creature with shroud can be enchanted this way.
- 1 May 2009If you choose to search your library but no applicable Aura card is left in your deck (or you choose not to find one), then nothing enters and you simply shuffle your library.
- 7 December 2018Any Aura card you find with Sovereigns of Lost Alara's second ability must be able to enchant the attacking creature as it currently exists. You need to check the Aura's enchant ability as well as any effects, such as protection, that would make it illegal to attach that Aura to the attacking creature. For example, if the attacking creature were an artifact creature with protection from blue, you could find an Aura with "enchant artifact," but you could not find a blue Aura.
- 7 December 2018If you choose to search your library but no applicable Aura card is left in your library (or you choose not to find one), then nothing enters the battlefield and you simply shuffle your library.
- 7 December 2018If the attacking creature has left the battlefield by the time Sovereigns of Lost Alara's second ability resolves, you can't put an Aura onto the battlefield this way. You may still choose to search your library just to shuffle it.