I concur mixing it myself would be the answer. If the RO
unit is a good one the mebrane is probably fine(TDS
meter will tell you for sure, just replace the other cartridges($20). A common error is that membranes need to be replaced yearly. If you have a good unit your membrane should last years (5-10). Get a 200g bucket of oceanic salt($50) and purchase a 40g holding tank with float shut off and water spigot for RO
holding ($85 installed by professional) and you will be set long term for pwc
and you inital fill for less than you would spend buying water.
Depending on how big your RO
unit is another option would be to call around on having RO
delivered. Around here it is like $.50/gal
delivered. That might be worth it for your inital fill up.