Chat with Dell live agent on line and give them your private service tag.  R14 case is offered in dark side of moon only as it appears on sale page.  I just asked and agent asked me for service tag.

Were you able to find out if you could purchase it separately? I overlooked it as well thinking it would’ve just came with it and now I want to buy a clear one lol

If they were smart they would sell the clear side and some of the other rgb add-ons and those of us that bought without would probably make them a little $ on parts, rather than alienating customers (no pun intended) from purchasing future systems, when they find out dell keeps upgrades strictly selectable at original purchase.