Sidewinder Sliver

All Sliver creatures have flanking. (Whenever a creature without flanking blocks a Sliver, the blocking creature gets -1/-1 until end of turn.)

Honor of the Pure

Agility

Enchant creature
Enchanted creature gets +1/+1 and has flanking. (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)

Cavalry Master

Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)
Other creatures you control with flanking have flanking. (Each instance of flanking triggers separately.)

Hivestone

Creatures you control are Slivers in addition to their other creature types.

Mtenda Herder

Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)

Grasping Giant

Vigilance
Whenever Grasping Giant becomes blocked by a creature, exile that creature until Grasping Giant leaves the battlefield.

Infiltration Lens

Whenever equipped creature becomes blocked by a creature, you may draw two cards.
Equip {1}

Mountain

Jabari's Banner

{1}, {T}: Target creature gains flanking until end of turn. (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)

Inspiring Veteran

Plains

Telim'Tor

Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)
Whenever Telim'Tor attacks, all attacking creatures with flanking get +1/+1 until end of turn.