Was the entire length of the wire burned, or just part of it? A loose or corroded connection will increase resistance in that area and cause it to heat up.
But if it was the entire length, then something else is wrong. Maybe a bad voltage regulator in the alternator? Just a guess... I'd test the alternator anyway (many parts places do free testing).
As for replacement wire, you might be able to find the wire, etc. to replace it at Home Depot. I've bought wire there for my car that actually says "oil reistant" on the insulation. They cut it to length for you. They also have heavy duty connectors that are at least as good as what was originally on there.
Be sure to check adjacent wires and hoses that may have been damaged by the heat as well.