Crowd source... VaultTec?
Probably not... at the scale of their Vaults.
Every once in a while, some kind of abandoned nuclear silo or mine or something does pop up for sale... but they almost all get snapped up by, you guessed it, billionaires looking to convert it into their personal apocalypse bunker.
For a well off layman? A group of maybe 10 or 20 such people?
Uh, basically, buy some shipping containers, cut out some walls and put in new supoort structures,, dig a big fucking hole out in the boonies, put your container-plex in the hole, fit it with a well/cistern, ventilation, filters, some kind of power solution, yada yada, bury it 2 or 3 stories deep, set up a ladder and manhole and basically airlock/decontamination entrance.
Probably gonna want an actual architect involved there.
Probably doable for ... comparable to the cost of house?
???
Only other thing I can think of is that there are actually a few, now long defunct, actual purpose built fallout shelter for maybe a couple hundred people, built back in the 50s/60s in various US cities... might be possible to use them as shelter, probably impossible to retrofit and refurb beforehand.
Or become a mole person and live in sewers/steam tunnels/subway tunnels, if your city has those.