I can buy a jump seat for my Tomos Quadro for less than twenty bucks from my Tomos dealer, but it is not a very comfortable looking affair. There is a jump seat on a Tomos just like mine that sits outside the Thunderbird Cafe in Enschede (NL) that has a nice one. I want to find one like it or change my seat into a longer seat so I can get my wife to ride with me.
Enjoy.