What could become the very best and the biggest nuclear blast and fallout shelter already exists in the Nevada desert about 100 miles North of Las Vegas. It is called the "Yucca Mountain Nuclear Waste Repository". It has never been used as a nuclear waste dump because of politics. With minor modifications on the air filtration systems and the addition of reliable clean water this system of tunnels could house perhaps 100,000 humans.
The other one that is not as big as Yucca Mountain but is already in existence and built for the exact purpose of surviving a direct nuclear blast is Cheyenne Mountain near Colorado Springs, CO.
Then there are the abandoned mines sprinkled all over the West. The salt mines under Texas and New Mexico come into mind. Also, the natural caverns that are tourist attractions today could become tomorrow's shelters with extensive modifications. With a little effort in stocking supplies, building air filtration systems, adding blast doors, many people can and will survive.