go to a welding supply store for anything 4awg or bigger... just make sure that it has a lot of strands... if the wire only has 20 or so strands stay away.. if it has more than you can count (talking hundreds/thousands) than you're good to go.
If you're an online guy and don't mind paying/waiting for the mail... here's a good place
https://weldingsupply.securesites.com/cgi-bin/einstein.pl?Next::1:UNDEF:OR:terms::PA#A1
price of 1/0awg is like $2 a foot.
Bout 5 years ago I picked up a gang of 2/0awg for like $1.50 a foot.
Welding cable is actually MUCH SAFER than normal car audio wire because it is UL tested, and has to survive in much harsher conditions. The only thing is that it doesn't look as pretty.