Corbin and Sargent are the only aftermarket seat sellers I'm aware of that offer complete replacement seats. Consequently, you're getting a one-size-fits-all seat that may or may not work better for you than the stock seat. They usually take a year or two to develop a seat for new models, so I doubt they have anything for the K16 yet.
Russell, Bill (Rocky) Mayer, Rick Mayer, Rich's, Kontour, etc. are custom saddle builders, in that they strip down your stock seat and rebuild it using better materials and years of saddle-making experience. Consequently, you get a seat that is customized to your own measurements and needs - height, weight, inseam, riding style, etc. Since they work with your seat, they can do new models quickly, limited only by how busy they are at the moment.
Sometimes, they'll have a seat pan in stock that they can rebuild to your measurements, then you send your old seat back in for the next guy (kind of like a core charge for some auto parts). But that's usually on popular bike models, and there are so few K16's out now that I doubt anyone has a spare seat just lying around.
Many riders opt to do a ride-in fitting, where they customize your seat, let you test it out, and get it all wrapped up in a single day. But that's not practical for everyone.
Other riders simply wait until winter and send their seat in then, thus minimizing down time.
Hope that helps clear things up.