These cacti are often available from cactus rescue groups, either as a participant on their rescues where you just pay the cost of the tag ($5 for low-lying cacti) or through their plant sales
I do not live in AZ - so joining the salvage effort is not an option. Do any of the resque groups mail plants?
With couple hundreds plants to go under buldoser there is obviously some number that people did not dig up because there is a limited room in the back yard, limited available labour, limited track space.. So may be selling them out of state will make sence?
But on a bright side plants dieing natuarly too and that is life. Environmental impact is likely to be neglectable unless it is an endangered species that grows here only - not the case with any echinocerei. And probably nobody digs chollas anyways..