So, I'm on level 28, played for just over 56 hours, and yet I have a Fat Man in storage with zero mini nukes.. I have caps out the wazoo but can't find them for sale or anywhere I explore..

What gives?

Super mutants often have a suicide bomber just kill it and avoid activate the bomb, you can loot it off the body. Sometimes, you can buy it from level 3 weapon shops at your settlements or sometimes at random shops.

More from here:

http://fallout.wikia.com/wiki/Mini_nuke_(Fallout_4)

Thanks!!

A nice place to restock would be at dunwhich, as the two nukes at the underwater shrine will respond every 2 days I think.

spread thru random locations , they respawn like trash does