Along the way I ended up with a Fat Man and a Cryolater. I'm just level 14 but can't find any sellers of the ammo. The ones mentioned in various walkthroughs don' have stock. I have found some cryo cells lying around but I've used those now.

Andi.

KLEO sells Cryo cells for sure, although you might need to level up a bit more before she starts getting them in stock.

get scavenger perk. I got more of those than I need. but Nukes are still hard to buy anywhere.

There are only a couple places that sell mini-nukes, and they typically only have like 1-2 in stock at any given time. They're quite expensive to buy too. If you build a level 3 weapons vending stand at your settlement, you can buy them there. But you've got a fair amount of leveling up to do before you can unlock the perks that let you build one.