Get yourself an RO unit, a 100GPD unit will fill your tank in a few days, buy salt and mix. Alternatively, find out from the local sponsors in your area if they sell RO water or premixed salt water and drive it to your house, bit by bit. Perhaps they have the facilities to deliver and offer this as a service?
In all Honesty, an RO unit is your best option. On a 600L system, it is a prerequisite IMHO.