This guide will tell you where to farm the Meso Relics in Warframe.
The first place you can go to farm Meso relics is Ukko in the Void, and it is a capture mission that can drop Meso relics or Neo relics as a completion reward. But keep in mind that you might not get Meso relics every run in this mission.
The next places are IO on Jupiter and Olympus on Mars; if you are newer, it is recommended that you run IO. Olympus on Mars is a disruption mission in which Meso relics can drop as a C rotation reward. You can get it from round 3 and up.
To get the C rotation rewards for this mission, you will need to defend all 4 conduits on round 3. On every round after 3, you only need to defend 3 conduits to get the C rotation rewards, and for the first two rounds, you need to make sure you at least defend the conduit or fail the mission. 
