The Pokémon Daisuki Club put up a mini-site called the Pokémon Mystery File (ポケモンミステリーファイル) and they announced on July 26th, 2017 that Solrock and Lunatone would be available to players of Pokémon Sun or Moon if they could solve the mystery of the Unknown characters hidden somewhere on the site. Players that solved the mystery would get the serial code OYAJI30YEARS to get these two Pokémon from July 26th, 2017 until October 31st, 2017.

Mini-Site Story:

Ancient ruins, time travel, alien rumors - There are many magical things in this world that can not be explained by science alone. The theory that the Pokémon familiar to us are deeply involved in all of these events is being discussed loudly these days. So our staff, in particular, validated some of the paranormal phenomena called "Wonders of the World" from various angles.

YouTube - Pokémon Daisuki Club - Pokémon Mystery File

Solrock and Lunatone Details:

Pokémon Info
Game: Sun	Game: Moon
OT: サクジ (Sakuji)	OT: サクジ (Sakuji)
Level: 30	Level: 30
Ability: Levitate	Ability: Levitate
Moves: Cosmic Power, Hidden Power, Solar Beam, Stone Edge	Moves: Cosmic Power, Hidden Power, Moonblast, Power Gem
Item: Comet Shard	Item: Comet Shard

Source: http://www.pokemon.jp/special/mystery/