This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
Odetics makes a thing called a DataTower that holds ~250 S-size D-2 cartridges. It used to be, but is no longer, sold through EMASS for use with the ER-90 (the Ampex/EMASS D-2 drive). It's a small silo that sits in front of one rack of drives. They also make an expandable library known as the DataLibrary, with a maximum capacity of ten petabytes(!) (ten million gigabytes). A robot handler runs on a track down an aisle lined with cartridges, and tape drives at one (both?) end(s) of the aisle. I think the aisles can vary in length, and they can be lined up next to each other and I believe cartridges will pass between them. (Note: since their acquisition of GRAU (above) EMASS no longer sells Odetics. I don't know if these are still available directly from Odetics and who you'd get to do the integration work. (rdv, from Dave.Barnes@fox.emass.com (Dave Barnes), 1996/3/22))