Where is the best place to order an LS swap wiring harness for a 5.3/4L60E C10?

Here is what the documentation says. Each point links to its source; details depend on your exact computer, harness and transmission.

• Gen III DBW: blue/green P59 PCM; Gen III DBC: red/blue 0411 PCM; 1999-2007 Vortec; 4L60E/4L80E: BP Automotive hookup wires: fuel pump, ignition, TCC brake, MIL, speedo, tach, fans, cruise. (BP Automotive)
• Gen III DBW: blue/green P59 PCM; Gen III DBC: red/blue 0411 PCM; 1999-2007 Vortec; 4L60E/4L80E: BP Automotive includes fuse/relay box, pump relay, fan controls, CEL, OBD2. (BP Automotive)

Confirm against the instructions for your exact parts before wiring. More: Harness and power in our tuning guide.