Hi there, take a trip down to your local auto shop, you can buy coolant in two forms pre-mixed or undiluted, I recommend buying a good quality undiluted bottle, most 500ml bottles will make 10 litres, you will only need 4 litres i think so its good for 2 flushes, clean your cooling system first, I always pull the heater hose off, put a hose in one end and have the other end drain out on the ground or in a bucket to flush the heater because these can get quite blocked. Replace any soft or cracked hoses as these can leak or if too soft can collapse causing overheating.