The list at the bottom of the page names all the Junk items in Fallout 76 that contain copper. This list is copied from the official game guide, which is not always totally accurate but gives you a good indication of what types of junk will contain the good stuff. The number in bold is the yield from each item.
As you might expect, anything remotely electrical is almost guaranteed to drop copper, but there are some unexpected entries in the list. Most weirdly Dehydrated Beef Stock, Diced Vegetable Mix and Meat-flavored Soy Chunks all give up copper, and if you’ve never heard of these ingredients it’s because they’re quest items for Feed the People, a repeatable Event that takes place at Mama Dolce's Food Processing.
Other oddities are the Ornamental Vase, Chinese Ornamental Vase, Lighthouse Souvenir, Magnifying Glass, Bunsen Burner, Beaker Stand, Trumpet and Bird Decoration, all items that I would not have guessed had any copper in them at all.
Also good to know is that the relatively common junk items Acetone Canister, Rusty Canister, Yellow Canister and Orange Canister are all good sources.
And if you’re wondering why Cooking Pot and Stew Pot have copper in them, IRL many fancier brands of cookware have copper-bottomed pots as this helps conduct the heat of the stove more effectively.
As usual, ore contains nothing useful at all since the metal has to be extracted using acid in a Chemistry Bench.
You can find copper mineral pits at the following Public Workbench locations:
The Forest: Billings Homestead and Wade Airport.
Toxic Valley: Grafton Steel.
Ash Heap: Charleston Landfill.
Savage Divide: Spruce Knob.
The Mire: Dabney Homestead.
Junk that contains Copper (yield in bold)
Three: High-Powered Magnet; Pre-War Lamp; Trumpet.
Two: Beaker Stand; Brass Miner's Lamp; Chinese Ornamental Vase; Lighthouse Souvenir; Ornamental Vase; Power Relay Coil; Sensor Module; Lamp.
One: Acetone Canister; Bird Decoration; Blue Table Lamp; Bone Cutter; Broken Lamp; Broken Light Bulb; Bunsen Burner; Cooking Pot; Dehydrated Beef Stock; Diced Vegetable Mix; Flight Data Recorder; Fuse; Hot Plate; Light Bulb; Magnifying Glass; Meat-flavored Soy Chunks; Orange Canister; Radio Jammer; Rusty Canister; Shadeless Lamp; Shadeless Table Lamp; Stew Pot; Telephone; Fuse; Vacuum Tube; Yellow Canister; Yellow Table Lamp.
None: Copper Ore!