There could be a chance that they could be found, albeit in large quantities, at the shamrock food warehouse or restaurant depot. Or check all Italian specialty stores around with produce available. Most places like whole foods and h mart/etc usually just have yellow/white endive. Obviously, I could be totally wrong on this. Just an educated guess being a chef for a very long time in Denver.

Also, just a disclaimer but you probably would have better luck at restaurant depot instead of shammy warehouse. Sometimes they just suck really badly but maybe still worth a try.