exmoor trim has them, but they're pricy.
quite a few other places have them as well, but again, the price is not what I'd consider affordable.
Mine came with 2, and initially I was looking at purchasing a set of 4, and putting them in.
I have now realized that if you intend to seat adults in the rear, two side by side, or diagonal mounted jump seats is the best configuration, as there is no leg room to seat 4 adults.
you can also keep your eyes on this list, or ebay...as some do come up for sale from time to time.