Dumpster Diving
On the subject of True Dumpster Diving, where you actually climb in and dig around, I can speak from real experience.
The best dumpsters are behind the small job shops in industrial parks. Bigger companies don't even want you on "their" property. Small shop neighborhoods have many small dumpsters in one area, quite often with the lid open so they can just toss stuff in. This allows you to go down the row and just peek in before you get serious.
Make frequent rounds, as the goods vary constantly. I scored many varieties of copper clad board at one favorite source. Larger companies will recycle their waste for salvage. I once collected some bits of wire at the phone company. A 12-inch piece of cable with 24 different color-coded solid wires makes a great handy dispenser. But a big mean old phone tech came out one day and made me give it back. He said "We sell those scraps. That wire is MONEY!" Jeez, excuse me for stealing 25 cents worth of trash.
In Irvine California, the friendly "Little Silicon Valley South", there are companies who put up hostile signs in the parking lot. They threaten to arrest you just for parking there. Forget about their dumpsters because they are under lock and key and video surveillance.
Good luck, and let us know the results of your endeavors!