Probably not, but you might try Steele Rubber Parts. Thats where I got the rear window gasket for my 67 Coronet hardtop. Of course, then it took calling every auto glass shop in the book before I found a glass shop with an installer old enough to remember how to install the glass and gasket on an old car and have it not leak.
The 66 300 2 door hardtop roof is kind of unique, so the back glass and gasket won't be the same as any other 66 Chrysler 2 door hardtop (and not even the same as a 65 300 2 door hardtop), or any other 66 Mopar for that matter. But, I believe the same 66 300 2 door roof line was available on 67-68 Fury III and Sport Fury 2 door hardtops as an alternate 2 door body style choice to the sports roof style roof found on all 2 door 67-68 C-body Chryslers and Dodges and most 2 door hardtop 68 C-body Plymouths. So if theres no listing for the 66 300 2 door hardtop rear glass gasket, you might find it for a 67-68 Fury 2 door hardtop back glass. But make sure its not the fast top (or sports roof) style rear gasket that would also be used on Chryslers and Dodges.