I've also looked for those exact stays or ribs or whatever we call them!
I was told by a U-Haul dealer they're a proprietary item supposedly made exclusively for or by U-Haul---which I find highly improbable.
I've contacted U-Haul about this and have not yet heard back---need to remind them I'm seeking some info!
A friend wants to insulate and cover his side walls of a straight cargo van and I've considered using standard metal house frame studs. Used only to support fiberglass reinforced paneling these would make stellar starting points. Easy to trim and form to the inside body contours and cheap too!