I also did a major search about 1 month ago for the gasket for my 68 charger. One guy I contacted told me that if I did find one to pay whatever they are asking if I wanted to drive my car this summer. (Factory production problems) Another guy said I was the 115th. caller that month. Anyway I went out on a limb and called an autoglass installer. They told me that their supplier had one gasket in stock. $105. And they told me they could have it the next morning. Had to drive 40 miles to get it because I was outside of their install range. Anyway the package was sealed and I was happy. Just got the window installed by a local glass company 2 days ago.

Search tips.
Old dealerships that might have NOS gaskets.
Mopar vendors that are not widely known.
Last resort, Junkyards that might have a good used one. I think there is some guys parting out B-bodies in the parts section.