Camping filters are definitely not the only way to clean water there are many others. For example: Home water filters, reverse osmosis, or distilling your water. But there are many other more scientific ways also.
Camping water filters can be purchased at your local outdoors store. I have found them at Mountain Equipment Co-op, Three Vets, and Coast Mountain Sports.
A camping water filter is for when you go camping but instead of having to take a whole bunch of water you already know is safe to drink, you can make any water you find naturally safe to drink by...
The best way to have clean water, is to boil it. Water purifiers are a close second, and although they still clean your water, they are not the best way to have clean water.