This guide will let you know where to get Adaptation in Warframe.
Adaptation can drop as an A or B rotation reward from the arbitration mission reward pools, which means if the mission is a defense, you have to stay for about 15 rounds then leave. For survival, you have to stay for about 15 minutes, and for excavation, you will need to complete about eight digs.
Stay for about 3 rounds for the interception, and you can stay for however long you want for disruption, but you need to defend 1-2 conduits each round for A or B rotation rewards.
It is recommended to bring the frames that can clear quickly at high levels or a frame with high survivability, Loki, Chroma, Nidus, or Rhino.
It may take you a few runs before you get it, and you can also buy it with a plat by trading with someone; it will not cost more than 50P (IMO).