Leyline Axe

If this card is in your opening hand, you may begin the game with it on the battlefield.
Equipped creature gets +1/+1 and has double strike and trample.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Flaming Fist

Commander creatures you own have "Whenever this creature attacks, it gains double strike until end of turn."

Fireshrieker

Equipped creature has double strike. (It deals both first-strike and regular combat damage.)
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Holy Avenger

Equipped creature has double strike.
Whenever equipped creature deals combat damage, you may put an Aura card from your hand onto the battlefield attached to it.
Equip {2}{W}

Duelist's Heritage

Whenever one or more creatures attack, you may have target attacking creature gain double strike until end of turn.

Genji Glove

Equipped creature has double strike.
Whenever equipped creature attacks, if it's the first combat phase of the turn, untap it. After this phase, there is an additional combat phase.
Equip {3}

Isperia the Inscrutable

Flying
Whenever Isperia deals combat damage to a player, choose a card name. That player reveals their hand. If a card with the chosen name is revealed this way, search your library for a creature card with flying, reveal it, put it into your hand, then shuffle.

Sigarda's Vanguard

Flash
Flying
Whenever this creature enters or attacks, choose any number of creatures with different powers. Those creatures gain double strike until end of turn.