Cheapest priced copy (Throne of Eldraine): $0.35 at Card Kingdom, near mint, their price list as of .
Shining Armor 
Artifact — Equipment · Common · Throne of Eldraine · #29
Flash
When this Equipment enters, attach it to target Knight you control.
Equipped creature gets +0/+2 and has vigilance.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
3 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Throne of Eldraine | 29 | Common | $0.35 | $0.49 |
| Jumpstart 2022 | 243 | Common | $0.35 | — |
| The List | ELD-29 | Common | $0.35 | — |
2 rulings from Wizards of the Coast
- 4 October 2019Gaining vigilance any time after the moment you choose to attack with a creature won't cause it to become untapped.
- 4 October 2019If there are no Knights to attach Shining Armor to when it enters the battlefield, it simply remains unattached.