Step #110 in the Expanded Vanilla list
The imperial skirt worn by guards and the imperial templar skirt worn by some legion officers are one of the “common but unobtainable” items, just like the shirt and helm of the gondoliers. The guards must get those skirts somewhere, right?
This mod adds a small quest to restore the supply of skirts. Talk to any legion member about “latest rumors”, then go to Darius in Gnisis and agree to help him. Then you'll have to walk around and talk to clothiers. Two of them in the empire-friendly towns can help you.