Suitable wiring for very low current mains equipment
Mains cabling doesn't just have to supply power to your lamps; it also has to be safe in the event of a fault. If your lamp short circuits, you don't want the cable to burn out or start a fire before the fuse blows (if you're in a fused plug country like the UK) or the breaker trips! So standard 3-core mains cable probably is appropriate. See also MandoMando's comment on the correct insulation. Or as Dave Tweed says, use low voltage lighting and a mains transformer (perhaps look for garden or outdoor lighting systems)

EDIT: if the rest of the installation is up to "double insulation" standards with no exposed metalwork, then 2-core mains cable would be acceptable, saving some space.