We actually have evidence the items probably won't be found elsewhere - there are already items that come from long ventures that aren't found anywhere else. (Young Water Lily)

Personally, I have been running 14 max level retainers through the level 100 ventures since a week after release (my main and my alt on different data centers) and have gotten exactly one partition, and one neon light. The drop rate is extremely low, as it was for the Young Water Lily.