Lua lenses can drop as a C Rotation reward on rounds 3 and up. This guide will tell you where to farm the Lua Lens in Warframe.
To get the C rotation rewards on disruption missions, on routes 1,2, and 3, you will need to defend all conduits. On Round 4 and up, you only need to defend 2 conduits for the C rotation reward.
It will probably take you some time before you get a lens because the drop rate is about 8-10 percent. So if you aren’t lucky, it will take you a few runs before they drop.
You would be lucky if you get it on the first few runs.