Both the positive and negative cable/strap should be at least 1 gauge, but 00 gauge is better/best for both.
Tractor Supply or a farm store or a welding supplier should have them. For the grounds, use a cable rather than a strap, if necessary.
Make sure all connections are clean/shiny. Use a conductive antioxidant copper-grease to minimize oxidation at connection points for battery cables and all ground straps.
