I'm up here too, and everybody I've ever asked looked at me like I have 2 heads when I ask them. None of the pet stores that I'm aware of, I bought gallons of distilled water at wal-mart for a while, and I have seen a few people doing the same. Wonder if they have fish tanks too?
i would watch out for bottled water. Make sure to test it beforehand. Even distilled water is commonly not pure as they often add phosphates to the water as a preservative. And that kinda defeats a lot of the purpose.
if all it is is "sterilized" then it is not the same. Unless something else was done to it, all they did was kill any micro organisms with wither UV or ozone sterilization. They didn't actually remove anything.
You can get something like the AP tap water filter. It's a DI filter and much less expensive, and will do just as good of a job.
Realm of pets has a cold sterile unit. Not to sure on the details of this thing. I used their water for a while with no problems. I even used distilled sometimes with better results than some of the places I would get my water at.
After just getting my first RO unit working a couple months ago I would definitely say it's worth it to buy one. Not only having the ease of it in your own house but I think quality control can be monitored a lot more. (i.e tds meter etc.)
I think that LFS and other places with ro/di use them so much they don't really pay attention to when they are suppose to replace the filters. Sometimes its better to be able to monitor you own equipment and be able to replace when needed for maximum efficiency.