Personally, I agree with everyone else here in that just splicing the cable is throwing money away.
I agree about the recommendation to buy a crimp tool and connector ends but as far as bulk wire is concerned, I've found that welding shops and some battery places will sell super flexible thick gauge (4, 2, 0) cable by the foot which you can buy on an as-needed basis as each project comes up.
If you are on a budget and need a long thick battery cable for a given project (say a battery relocation), E30s have their battery in the trunk and are wired with 1/0 cable (or something in that range of thickness).
PS. If your starter cable is getting frayed, don't neglect proper grounding if you are having a hard time starting the car. On my tbird track car, I ran a separate ground cable from the starter mounting bolt back to the battery.