In other missions the landing craft parts drop from rare containers.
But the parallax parts actually drop from Reinforced Carrypods, which are a lot more common indeed. In case you're referring to this container.

The rare containers on Zariman missions that can drop forma, are the (Ancient) Molted Cask.
From my experience they seem to be similary rare.
As you said, if you're following the kiddo, and are looking for items on the map. You'll find the rare containers more, compared to when you blitz through completing the mission objective as fast as possible.

Thanks, got the tablet and somachord this time as well when joining the duviri experience.
Maybe ithey don't spawn in the time limited kullervo mission, or it was just a bug ...

Find the 3 statues on the island, and rotate them so they face this room.