The bag I have (and love) for my XPS13 is a Targus messenger style bag, found in new/unused condition for $5 at a garage sale.
From searching images of it online, it seems to be
(very similar to) a TSM101AP-A7 bag, and it does fit my XPS13 absolutely perfectly, with lots of smaller pockets for various accessories and other electronics. Nice and compact.
http://miyanjigroup.com/product/targus-tsm101ap-a7-netbook-messenger/https://www.amazon.in/Targus-TSM101AP-12-inch-Netbook-Messenger/dp/B00NHMSQ9UOf course, years later now, and it is no longer generally available.
