Try NAPA-last I checked they had a listing for these. And if all you need is the rubber cups, they're an American size and a generic cup will fit.
Keep an eye on ebay if you're not in a big yank (there was a 4 wheel NOS kit for $15 recently that didn't get a bid)
Some likely expensive suppliers: www.ipdusa.com
Just bought a '53 PV444 and dragged it home. Split rear window!