Cheapest priced copy (Oath of the Gatewatch): $0.35 at Card Kingdom, near mint, their price list as of .
Strider Harness 
Artifact — Equipment · Common · Rivals of Ixalan · #183
Equipped creature gets +1/+1 and has haste.
Equip {1} ({1}: 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 |
|---|---|---|---|---|
| Oath of the Gatewatch | 167 | Uncommon | $0.35 | $0.49 |
| Rivals of Ixalan | 183 | Common | $0.35 | $0.49 |
| Scars of Mirrodin | 207 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 19 January 2018If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won’t be able to attack that turn. This means that you can’t use one Strider Harness to allow two new creatures to attack in the same turn.