Don't have a source - but a possible solution depending on how the original piece fits (can't recall off the top of my head right now - sorry, it's been awhile since I had them apart). Buy some black rubber tubing, and either: 1.) Carefully cut it in half (lengthwise) with a sharp razor and you'll have two U shaped rubber gaskets that you can push in the channel. or if that's incorrect 2.) Don't cut it in half and press the whole tube into the channel.
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Jeff B.
'75 Alfetta Sedan (rosso)
'69 Lancia Fulvia Rallye 1,3 S (rosso)
'92 Ducati 900SS giallo gone but not forgotten
'79 Alfetta Sprint Veloce(luci del bosco)
'77 Alfetta Sedan (nero)
'76 Alfetta Sedan (argento metallizzato)
'86 VW Scirocco 8v
'72 Honda CB500/4