I scoured Napa to no avail - I'd ask Roger Koop, he might know where to get some. Haven't seen them sold by themselves (i.e. without the lights). As a last resort, I supposed you could chop the little socket off and drill a hole in the middle of the ones you found - seems they would still seal correctly on the perimeter.
Not OEM but these might work???
http://www.empirechromeshop.com/p-52...yABEgKVAvD_BwE
I'd ask Roger first
EDIT: might also contact seller of these kits and see if the gaskets are a match, but I'm guessing they are like the ones you found already
https://www.ebay.com/itm/5-For-73-87...LH_TitleDesc=0 OOPS - disregard - I just looked at ALL the pictures - not what you're looking for... sorry.