Have a look in the phone book for a firm that does auto electrics or alternator repairs, should have anything you want & will crimp lugs on for you at little cost.
You can buy it by the metre at lots of places. Or, if you want very short lengths, Acorn Engineering sell ready-made link cables with the terminals fitted.
Try a welders supplies. they do flexible cables with enough insulation for 12V and can crimp heavy terminals on them.
Buy them the length you want. What you need might even be an offcut.