I bought from this place they will have everything you need!
http://www.vehicle-wiring-products.eu/VWP-onlinestore/cable/batterycable.phpAll I did was buy 2.5m of red copper core bolted it on the positive terminal, feed it through rubber gromit on fire wall, dug up the carpet & trim, then feed it through the boot. The negative grouped terminals bolted it onto the gear box. Then fed about 10 inch long black cable from the negative battery terminal right down the chassis where I made sure the paint was off then bolted it to the whole body. Some people drill a hole in the boot down to the rear axal or underneth where it can be hidden. I didn't want to drill any holes so found a hole on the corner of the boot near the central locking pipes are visable. Best to buy some Eye lets so you can solider directly to the core cable you get better continuity with some silver solider.
You need an isolator for the positive feed cable depends if you want two cable method being the break after the firewall the second cable is simply connected to the battery.
My cables where thick but still felxible I would email those guys as to what available sizes they have and is suitable for you.