I will be re-wiring my entire car soon during my restoration. Yes, Lectric Limited is a very good source. Be careful. Most everyone, including Lectric Limited sells new harnesses and NOS harnesses. NOS, standing for New Old Stock, is a new harness which has sat on the shelf for 35 years. It will have the same insulation cracking problems that your existing harness may have. LL sells all the harnesses year specific.
Since my car is so far custom, very little of the original harness for the front and rear applies, so I am making my own. Different (halogen) headlights in a different location, LED front turn signals in a differnt location, C4 backup lights, no clearance lights, sequential rear turn signals, Cougar tail lights, electric fuel pump, etc. The engine harness is also totally different, alternator with internal voltage regualtor is different (yes, the '68 actually came with a BS external voltage regulator that constantly failed, overcharged and burnt up the battery), HEI distributor, electronic fuel injection, electric radiator fans (you get the message). I will be using all Ancor marine tinned wire. Costs more, but it has never corroded on my boat, so I believe it will be fine on my car. I did purchase from LL the interior harness for about $425.00, if I remember. I bought the 1972 harness, as I am installing a 1972 steering column and this harness will have all the correct plugs. I needed the fuse box as well. The fuse box cannot be purchased anywhere I could find unless you buy the whole interior harness, not even thru LL. Since I know they have new fuse boxes for their harnesses, I asked them to sell me the fuse box and they said no. In fact, they won't sell practically any plugs either. I tried to purchase the '68 specific plug for the brake light switch, as '68 Vettes have a different brake light switch than any other C3 Vette. They said no. So instead of splicing my old switch into the new harness, I converted my brake light switch on the pedal bracket to the newer type.
Tim: My original harness was a disaster. I know now why '68 Vettes had so many electrical problems. Not only did I have corrosion under the insulation, I actually found unclamped, unsoldered twisted wire connections under the tape, and they were corroded to death. Check all of your wires for insulation cracking or brittleness. My wires were so stiff in some locations, if I cycle bent the wires a number of times, the insulation cracked. If you are going to replace any of the spade connections that go into the firewall (engine side) plug, I have a few dozen that meet OEM specs and can show you where to get them.