Unfortunately, Vintage Trailer supply does not carry the window gaskets for your model year, but they do carry Vulkem and, I believe, Parbond. Read more about these two sealers in this thread
. Chances are, though, that your gaskets are not a big problem at this point. If you do decide to replace them, I posted a few tips at my website. Your '68 is quite similar to my '67.
posted some excellent information on sealing window hinges of your Airstream's era with RTV. I wish I could locate that post because the window hinge is the only part of your Airstream where RTV is an approved sealer.
Start with this thread
, and go from there.