Looks like its just spring steel that provides support and a bit of give to the backrest. I doubt if you can buy them, but an upholsterer may be able to fix it. I would get a similar sized piece of wire and join them back together, making sure no sharp ends are pointed in, but preferrably towards the back or to the sides.
You could also chech a wrecker for another back, but many would sell the entire seat, some you go in and get the parts you want. Not sure how its secured, welded, wound, or clipped in or bolted on, the last 2 would make it a lot easier to change without damage.