Here's an update: I found a total of (4) Virgil Crow cachets with the American Indian Plate Block (Scott 1364). Considering the link you provided suggests that he made no more than 20 or 30 of each cachet, it means there must be very few left on the market.
Likewise, your link also allowed me to uncover a Virgil Crow cachet for the Christmas 1968 stamp (Scott 1363) as shown below:

I'll have to keep looking out for others.