Can you buy big buss bars to hook battery cables together? Or do you make them yourself.

you can do it either way. googling buss bars will show some places that have them. depending on your location you may even find a place near you that sells them. keep in mind that it should be the equivalent to or better than the gage wire you would have otherwise used with my recommendation of twice the current capacity by halving the resistance you would have normally wired for.