Robert Gilmore will probably chime in here with his recommendation for extra-pure virgin oxygen-free 12AWG copper wire (not that there is anything wrong with that).
I however think it is a bit of an overkill. A plain 14AWG or 12AWG (if your layout is really large with long wire runs and your trains draw lots of current) zip cord will be more than sufficient. Just Google 14AWG (or 12AWG) zip cord and you'll find plenty of sources. I prefer the red-black zip cord to the all-black zip cord used for low voltage landscape lighting. The red/black wire insulation is thinner (easier to work with) and the colors make identification of the polarity easier.
Thank you Peter. If I were to do it again, I would use 14AWG extra-pure virgin low-ox copper speaker zip wire in red and black. What I went with was identical speaker zip-wire but in 12AWG. There is some good evidence that signal loss is much less in 14AWG wire than 12AWG due to the effective diameter of the fine multi-strand physical characteristics vs DCC signal type. Don't ask me to 'splain it, but the evidence presented by an electrical engineer was pretty convincing.
As a bonus, 14AWG premium grade wire is cheaper than 12AWG.
All that said, on my present 36' X 20' portable layout, I have no evidence whatsoever of signal loss anywhere.
I found a helluva deal on 300 ft rolls of my wire on eBay, so I bought a couple of 'em. Much less expensive than the crap I found at The Home Depot and Lowes, made for outdoor lighting that, when stripped, was pre-oxidized...badly so..like green.
I agree that the metal purity is probably "overkill", but price-wise, it wasn't. I prefer "overkill" on complicated to install items like layout wiring anyway since it's just as easy (or easier) to install, and I can be confident that as my modular layout grows to its ultimate 30' X 50' size, I won't have to rip it out and replace it with better bus cable.
Peter is probably correct that the difference between "just" copper cable and "pure, lo-ox" copper cable is incremental. However, the bigger the layout, the more any potential problems will occur.
Better safe than sorry..
Cheerio!
Bob Gilmore