How about:
If you FAIL at Crafting, you get Junk. (And there should be a Crafting Success Rate)
If you have some Spare Weapons/Items you can convert them into Junk, Wood and Electronic Parts (depending on Science), but not Metal Parts.
Junk should also be found on different Tiles, instead JUST City Tiles. How about some of them at a "Random Gasstation" or Cars on the Map?
Also instead of searching for Junk on the Ground.. How about finding them in Cars on Random Encounters? There are often Cars on City Tiles, and not just one.
It should also have a Random Rate of Junk to get from it.
Like 5 from a Truck, 4 from a Highwayman, 3 from.. whatever. You get the point.
Also, any way to get Nuka-Cola?