While I was stationed on Okinawa with the U.S. Navy, our U.S. Naval Security Group Activity (Hanza) was "deinstalling" an entire room of equipment. Into the Dumpsters, they were throwing away the interconnecting cables. Had I waited overnight, the local Okinawans would have Dumpster-dived for the cables and sold them for next to nothing for the copper. I ended up with around 200 feet of 50 individually shielded pairs, each with a bare drain wire, each having at leat 95% shielding. All wires and shields were tinned copper. Wire was #22 AWG solid and each was covered with solid-color insulation with a decently-high melting point unlike the cheapie PVC. And with 50 pairs, I ended up with all ten of the standard colors, black through white, 0 through 9. I could use the shields for all sorts of heavy-current bus uses or pull together several strands of braid 12 feet long for making some super-current automotive jumper cables after adding some high-quality clamps to the ends. The bare wire is great for PCB jumpers and short jumpers (power and ground connections come to mind) for solderless breadboarding. Needless to say, it's a lifetime supply of wire for me!
Ergo, an excellent source of such wire is to see what the telephone companies leave behind or throw away. Check them out to see if they'd part with some. No sense buying if it's free! Both telephone and CAT-5 cable tend to be #24 AWG vs. #22, but still plenty useable for breadboarding or hard wiring.
Dean