Most hardware stores have a selection of springs. At least the local one does and I'm hoping they have ones that will work, as my seat springs are missing as well. Maybe Otto will be kind enough to post the dimensions of his springs.
Although I may never move my seat once it's installed. I don't know if I'll ever let my wife (a full foot shorter than I am) ever drive my truck once it's done.
[on edit] found the part numbers (and some dimensions) in the parts manual (pics attached).
