The only one I know of is Wunderlich, someone on this forum has it. Based on their post I wasn't impressed, certainly not for how expensive it is. There's the BMW Comfort Seat which is much better than stock. Reviews are generally favorable, though it didn't work for me and I've got one for sale in the classified section.
The only other options are custom and semi custom. Semi custom is the way I went, I bought a seat cover made for the bike and reshaped my stock seat to my liking. Full custom would involve finding a local seat upholstery shop that will reshape your seat and fit a custom cover to it.
Expect to spend anywhere from a couple hundred on up for these options.