Hi Rick,
I replaced this seal in my airplane back in June of 2009. I purchased it from Cessna at that time. My contact at the time was Melissa Clark. Her email at that time was
MClark@cessna.textron.com. I have no idea if she would still be around all these years later but what the heck, send an email to this address and see what happens. I purchased enough of the S2171-1 to do all the side windows. My recollection of obtaining this material at that time was it was surprisingly easy and not very expensive and Melissa was very helpful.
Oooops, re-reading your post, I see the part numbers do not exactly match, but you could still attempt to reach Melissa. I'm guessing these seals are similar.
Good Luck,
Ed