No substitute for the right equipment.
Look for used on local Craig's list and local reef club website.
More importantly, turn to local reef frineds. (Join a club if not in one already.)
And finf a close reefer friend who can supply you with water. later, when you get your system and his/hers goes out you can return the favor.
Even with an Ro/DI system, we all experience at one time or another - no cartridges becasue you forgot to order them, or the membrane goes out, or something. So, make a contact/friend and have a backup.