The pin is Stanpart #607823. Oddly, Canley Classics in the UK shows it as not available, but several other UK vendors list it, as do Spitbits ($5.15 each). But the #607824, the hinge, is only $27.50 from The Roadster Factory ($33.00 from Spitbits); it might ultimately be easier to change a hinge than just the pin?
Unfortunately, you do need to make sure that's the problem -- a worn hinge. If it's only that, you might be able to compensate temporarily with some adjustment. But it's also good to be sure that the sag is not caused by structural problems in the "A" post and/or sill due to either fatigue or rust. /ubbthreads/images/graemlins/mad.gif