REI has the Therm-a-Rest Z-Seat Pad, it's yellow. The blue foam bum pad Teslinhiker shared is on the MEC.ca website. Either would be nice to have, but probably not necessary in an urban walking kit.

Dagny -- you might think about one of the pads for your sister's winter car kit. Curious -- what kind/size FAK is in that car kit? You might want to generalize that kit rather than keeping it specifically winter. Just a thought.