If you go to the main K12/1300GT (Next Gen)
forum page, you'll see a Search this Forum link on the right just above the thread listing. Click on it, and type in Sargent and you'll get lots of results.
Here's what I wrote in response to a similar question as an example:
You can buy an off-the-shelf replacement saddle from Sargent or Corbin, but they both go with a one-size-fits-all approach and simply use a much harder, flatter shape that works for some folks but not others.
But if you really want to be comfortable and completely eliminate squirming around on long rides, then you should look at having the seat redone by a proper saddle maker.
The top guys by reputation, longevity, and comfort seem to be Bill (Rocky) Mayer in Ojai, CA, Rick Mayer in Anderson, CA, Russell in Shasta Lake, CA, and Rich's in Kingston, WA. I'm not sure why they all seem to be on the west coast though.
I've had great luck with my Rocky Mayer and have put 95K miles on it in 28 months. If you are larger and heavier (240 and above) I'd probably recommend Russell. They have an internal spring system that helps distribute the weight better for larger riders. Plus they offer a built-in driver's back rest option if that's something you're interested in. Another choice for a back rest is Bakup which works with any seat, and is what I have.
If you can do a ride-in appointment that's preferred, but these guys do 80-90% of their business through the mail. Occasionally you have to send it back for a rework after putting a few thousand miles on it, but that's a minority of cases. But a ride-in is a great excuse to get to ride some awesome new roads.
Some folks will claim to have good luck using beads, sheepskin, or an Airhawk, but those are cheap, temporary solutions that are simply stuck on top of the seat. That's like claiming the original Concours is just the same as a GT because it also happens to have bags and a windshield.
Getting a proper custom saddle isn't cheap, but then again, neither is a BMW.
So it's up to the individual whether spending the money is worth it or not. For me, it definitely was.
'13 Dark Graphite Metallic K16GTLD, 24K miles and counting...
'09 Magnesium Beige Metallic K13GT, 60K miles miles and counting...
'02 Mauve Metallic K12LTC, 106K miles and sold
BMWLT#145, IBA# 366, MOA# 111996, SCMA# 24032
All lower 48 states plus Alaska on the K13GT in two weeks . . .
Some people see the gas tank as half empty. Some see it as half full. All I care is that I know where the next tankful is coming from...