A cheap AC/DC converter is the way to go. I actually have a 12V and 5V ac/DC transformers (aka wall warts) that plug into a strip on the underside of my layout. I then run a bus wire around the layout from each so I have access to that DC power from virtually anywhere pretty quickly. I bought 8A wall warts though I will likely never need even close to that I didn't want to ever run into a situation where I needed more power.
