Plug wire grommets commando valve covers

Here's one place...

Mega Parts USA - Reproduction, NOS and Used Mopar Muscle Car Parts

...and another...

1967 Dodge Parts | MD2401 | 1966-68 Mopar Small Block Spark Plug Wire Separator - Valve Cover Snap-On | Classic Industries

...and yet another...

Spark Plug Wire Retainer 66-68 A B C Body Cars 273-318