Echinocereus acifer Milpillas, MX 1600m
Echinocereus acifer from Milpillas, MX 1600m is sometimes lumped under Echinocereus polyacanthus var. densus. This is the furthest north location I could find for this species and it very well could just be a form of polyacanthus but either way these should take better cold than most forms on the market. I have tried acifer in the past where it survived mild winters of 0F but did not survive -17F. However that was with further southern locations, these should do much better.
I do not guarantee these in a zone 5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 90 days before 1st hard freeze.