A sample of the wiring seen from below. Multiple leads come down from the track and are connected to the large bus wires with run and tap splices
The main power supply wire I used is 12AWG stranded (not solid) wire I bought in rolls at Home Depot
The smaller wire is I think called hookup wire and can be obtained at Radio Shack
I use two types of connectors for connection track etc to the big power wires I use 3M Scotchlock Self-Stripping Electrical Tap Connectors 905 (part 054007-06128), these connectors are sometimes called tap and run connectors and can be closed with normal pliers.
For connecting small contact and turnout wires I use 3M Scotchlock UY connectors, also called butt connectors and the UY ones handle wire sizes 22 - 26 AWG.
I bought both type of connectors mail order via the internet.