That's not quite correct question to my mind, the correct one would be - If you don't have RO/DI unit, why don't you get one?
Seriously, it's only (comparing to cost of almost Anything in this hobby) $100 on e-bay and will save you ton of time (running to grocery store) and in not so long run money too.
Other than that - the tap water in Redmond for example is only 35 TDS, which is surprising, but might work for FO tank. You might wanna check your water and see what you need to do with it in order to make it acceptable. (But get RO/DI is my advice).
you can buy large bottles of ro water at any of these bottled water co's. they well even deliver on a schedule if you want. I agree with solov on getting a ro/di unit they just don't cost that much. You may only have 35 tds but what about phosphates?
I buy the Glacier self serve RO water from Safeway, here it's $.39/gallon, I have to buy 15g/week. I keep attempting to buy a RO/DI, but each time I save up some tank money it ends up eating it in other ways. They are cheaper in the long run.
You can get water delivered, but for what it costs, it's more than worth your time to drive to Safeway IMO.
my opinion is just force yourself to get an RO unit. for almost 3 - 4 months after i set up my tank i bought water from either the aquarium or from a grocery store. ugh! what i spent on water in those few months i could've bought an ro unit. now i have better control over the quality of my reef tank water as long as i don't get a bad batch of salt mix
Don't forget cost of filters/membrane. I just spent 150 clams on them. It adds up . Also curious as to when everyone replaces their filters. Is there a formula out there to figure out when to replace? I'm replacing the first two filters every four months or so when they look dirty but what about the membrane and silica filter? It's much tougher to determine when to change them. Any way to do this without a meter?
BTW, I do believe Gordo does have an RO unit. He's looking for an interim solution while his is on the fritz. Having said that, yes, I agree, getting an RO unit is a great investment for your tank - and your drinking water.
the di filter well change to a copper looking color from its normal dark color, about once a year normally is enough. the membrane should last for years if you flush it every couple weeks which cleans its surface and when it goes bad your output well go down, other than that it is working.