Marine grade tinned stranded battery cable from 6 awg up to 4/0 Awg size. This wire is sold by the foot. Wire by the foot is sold unspooled. Most wire is also available in 100 ft rolls which are more economical - just ask for details.
What is so special about marine grade battery cable? It has to do tough duty in a demanding environment where heat, vibration, water, oil and a corrosive salty atmosphere may on occasion be found. It needs to be flexible, stranded, and it needs to be resistant to water and chemicals. I sometimes find welding cable used on boats but I have seen the insulation disintegrate because it came in contact with oil or fuel.