Connectors (OEM): Mouser, Ballenger Motorsports, KSV Looms
Connectors (MilSpec, Deutsch, other fancy shit): ProWire USA, KSV Looms
TXL Wiring: WireBarn, OEM harnesses from eBay (sometimes OEM harnesses are takeoffs and CHEAP and I scavenge them for perfect matched wires)
MilSpec Wiring: ProWire USA
Sheathing (split loom, generic heat shrink): eBay
Sheating (DR25, fancy shit): ProWire USA
I've probably spent 10K across this collection of vendors on electrical stuff, no complaints.