Most welding shops carry many sizes of welding wire and sell it by the foot. #3, #4, #2, #1, and# 2/0 and #4/0 is all available at my local shop and they also sell heavy duty crimp on ends. In my shop if you buy the cable and ends they will crimp on the heavy duty ends for free. Welding wire is very finely stranded making it perfect for battery cables and its outer jacket is oil and grease resistant. Put it, after made up, in split loom and they will last as long as you own whatever it is you are working on. Always use larger cable then you thought you needed. For auto cables I use 2/0 but for a 3000 watt inverter I only use 4/0. Get yourself ready for sticker shock... Copper is really high right now on the spot market.
-Paul R. Haller-