Your best bet is to talk to Graeme Payne at
https://www.classicbristolcarpartscompany.co.uk/
You need an extruded rubber profile which is not dissimilar to others on the market but the others aren’t quite right. Bought as a long length and cut to suit.
I found it also helped (on a 410) to add a plush seal, trapped behind the headlining fixings, that bore gently on the inside of the chrome frame.
It’s worth doing the rear windows as well, they use a different profile. Apart from anything else a new seal makes the over centre catches much more positive.