Walmart sells battery terminal connectors...
As a rule of thumb... If the battery cable (positive or negative side) is stiff (no longer flexible), or makes cracking noises when you try to bend it, you need to replace it.
When making your own, I have heard the best is making them out of arc welder(can't remember for sure which welder), cables. They are made for high amperage draws, are very flexible, and contain a lot of small individual wires (the fewer the wires, the easier for it to fail) Also when making your own, it is easier to use bolt on contectors, but I'd recommend using the brass solder on connectors... not sure where to get them, but any diesel repair shop (semi tractor, not oldsmobile type diesel) will have them. strip the end of the cable, slide a heat-shrink tube up it sufficiently far from the end, clamp the brass connector into a table vise, heat them up while getting them about half full of solder, then stick the stripped bit of cable in and hold it til it sets. once it has cooled, slide your heat-shrink tube down over the connection and heat it up... this connection will be trouble free for most of the life of your rig.