As I reccommended in another thread, you can buy hefty 5 gallon jugs and go to grocery stores and but it for pretty cheap (like 30 cents a gallon). You can also use it for drinking water (which I personally prefer way over tap water.) You need to be careful though since some units at stores have clogged filters and need maintenancing, but it could be a cheaper short-term solution.
We used to have a water store around here called Water-to-Go that had an EXCELLENT RO unit (the store was devoted to clean water) and it you could get it for about 19 cents a gallon. Unfortunately it shut down due to the sudden death of the owner (pretty sad really, knew him for about 9-10 years).