Blizzard is adding two new kilt options for transmog in Patch 11.0.7, alongside matching leggings and slippers!
The War Skirt transmog comes in numerous colors, so it's likely available via the Trading Post like other simple transmog pieces such as tabards, Vagabond cloaks, and sarongs.
The War Skirt set contains skirts in two different lengths, leggings, and slippers.