Where can I get rarer ammo in Fallout 4?

Solution 1:

The Commonwealth Weaponry store run by Arturo in Diamond City stocks all kinds of ammunition, including fusion cells, plasma packets and cryo ammo. This is the same with the vendor in Goodneighbor.

For finding this ammo out in the wild, in general you're going to want to be fighting against the enemies that use each kind of weapon (so for example, you'll find lots of ammo in the Institute and fusion cells drop off synths).

See also: Where do you get Ammo for Cryolator?

Solution 2:

Having the scrounger perk is always helpful for finding the less common ammo types and of course you can usually buy most ammo types from merchants. For the Cryolator specifically think you have to primarily rely on merchants. I have asked a few people and they said they haven't seen any cryo ammo drop from random boxes so there is probably little chance or no chance at all of finding it while scavenging.