I might add that bottled water is not a very good solution. Many bottled water brands are worse than tap water - and less regulated. They sometimes contain high amounts of fluoride.
The best filter is a reverse-osmosis system. Or you can get a countertop fluoride+carbon filter that attaches to the kitchen sink, like this one...
H2oFilters.Com CounterTop Water Filter with Doulton UltraCarb Ceramic Filter and Fluoride Filter - Fluoride Reducing Filters
You don't want to drink tap water anyway. There are many other issues beyond just fluoride. Having a decent filter is a good idea.
As for cavities, if anything fluoride creates more. The best solution is a healthy diet, low on processed starches. Your teeth absorb nutrients from your food. This is what prevents cavities, not adding industrial waste to your water.