If you have any 4awg cable laying around from a stereo install, or find an amp install kit you can use that stuff. It's a little easier to come by, easier to work with, and far cheaper. For the length of ground cable you need 4awg should be fine. I think I bought some 2awg marine cable from ebay when I did it last (~12 years ago). Spent like $50 on the stuff and had way too much left over.