66 Valiant Rear Glass Rubber

A quick search didn't turn up anything at Steele, Metro or Softseal; but you probably knew that...
MoParMall lists them up to '65, not sure what the difference is for '66:
Molded rear window seal for 1964-66 Plymouth & Dodge A-Body (moparmall.com)
You could also try Quirey, call them as the website does not have a complete inventory:
Quirey Quality Design Co. (business.site)
My "go to" list for tough to find parts:
1962 to 1965 Mopar Restoration Vendor and Parts Sources (ornocar.com)
And as a last resort, a good autoglass shop should be able to get the profile you need in bulk, and cut it to size and heat seal (or butyl) the corners or ends... Steele, Metro, etc. should also have it in bulk.
Good luck!