How about a sealed bottle of distilled water? You should be able to pick one up at the grocery store for a few bucks, bottle included! :-)

That said, I have to admit I have some Nalgene bottles in my car. I just use tap water and every once and a while stick them in the dishwasher.

I like the Nalgene bottles for the car and hiking since they are very heavy duty and take a fair amount of abuse.

I'm certainly not an expert, but I feel pretty safe only re-cleaning and re-filling each year.

-john