for rinse - your garden water hose pipe is best (if its clean fresh water), for final fill buy some RO water, same stuff from bottling places.
Dont buy premix - as it becomes a very difficult task to control the mixture when filling from zero after flushing. The easiest way to do this is to first know the cooling system quantity. If e.g. its 6 litres, you need to buy 3 litres concentrate.
measure the expansion bottle capacity too - if its 1 litre - then you need to use half litre coolant concentrate for that and 2.5 in engine
after flushing everything and packing it back up, pour the 2.5 litres concentrate in the radiator cap, then the rest should be topped with distilled/RO water. once its done, close off cap and start engine, slowly let it cycle in the system - the pump will mix the coolant.
After this your system will hold a true 50% coolant mixture. - If you have to do any repairs that involve draining it, collect it in a clean container and reuse it.
