I use 14.8v 4S (4S = 4 cells) Lipo batteries in my main locomotives but if you are planning on big locos and long trains you might want to consider 18.5v packs. It depends as they say. Unlike Nicads or Nmh batteries, Lipo cells are 3.7 volts each so 4 gives you 14.8 volts. In smaller things like critters or rail cars I use 2 or 3 cell packs. I order packs from Hobby King. I like their ‘lipo finder’ page. You can search by physical size as well as cells and amps, so you can measure the innards of your loco first, then get a pack that fits.
I generally stick with Tenergy Packs although I have a few others that have been fine. It is important to have a quality lipo battery charger and treat the packs with respect. They can dump a tremendous flow of amps if they are shorted. It can melt wires and start fires and will ruin your day. ALWAYS use a fuse in your power circuit hookups.