Probably the best place to start is Witmer's Coach Shop in Pennsylvaina. They seem to have almost every part to fix or improve your vehicles. They are Amish, but great to work with even if we don't know what we want!! I think you can order a catalog and look at the pictures of items. Might have to pay for the catalog these days. They don't have a website that I know of. They can ship you anything they carry!!
Witmer Coach Shop
1070 West Main Street
New Holland, PA 17557-9380
I don't know what kind of cart you have, so not sure what kind of seatbacks would fit. I am "pretty sure" you call the part holding up the backrest, seatbacks. You would want hinged seatbacks. You can make the backrest part yourself out of a wood board nicely finished. You can add some padding to match the seat or leave unpadded.
I will say that ANY cart's comfort improves with a backrest!!