You 1) buy ro water for the lfs or a local wallmart. and carry it home is whatever cantainers you can find. 2) you pay $150-$200+ for a ro/di unit for your home. Collect the 20g per day water generated in whatever container you can find. Just be sure the containers are clean and free of any polutants, especially metals like copper. According to members here you mix up the water and salt and aerate it for a day or so. Then add it to your system.
With either system make sure the equipment is properly maintained and the filters are in good shape. If your own unit and the filter are bad, replace the $80 filters.
Or you use your tap water, mix in the salt and add it to your tank. the choice is yours.