I'm convinced you need to craft them. I have been searching for supplies to go to the deadzone to get my hands on advanced crafting supplies, so I haven't checked in some time. You need explosives, advanced crafting, electronics, defusers, GPS and some other things I think. I partially forgot. Hope this helps, man.

I've found two so far, it'd seem you can get them from locked crates.

The ones I found where on the Backwoods Diner's locked room that has a couple of them, you'll need 4 crowbars, one for the door, and 3 for the crates. Alternatively I think raiding gear can work on padlocks as well... getting sick of needing a crowbar that takes 4 slots to then get 1 bottle of gunpowder.