I get cable for such purposed from Vehicle Wiring Products ; www.vehicle-wiring-products.eu
Have used them for years and service has always been good although prices are ex VAT and less P&P so a relatively small order somehow gets expensive.
Make sure you get a good big cable - I would go for at lease 8.5 mm.square or even 10mm square rated at 60 or 70 amps respectively - you won't need more than a few metres. Make sure you get a really good join onto the existing cable if that's what you were planning but I would consider replaceing the entire length.