For the do it yourself-er, I suggest trying a few of the simple science projects to get yourself familiar with the kind of filtration you can expect from materials you can gather around you. After you see how the water may still need boiling or chemical treatment before drinking, Then you could buy a well made filter and try to construct your own berkey type system.
I really like this site called When Technology Fails, by Matt Stein. He has authored several books but the post on filtering and purifying water is brilliant. His chart for disinfection and sterilization is a lot of information in an easy to grasp visualization of the facts. Read the post, or buy the book.
For pictures and video, try http://ronmauer.net/blog/?page_id=178.
My eldest took to making his own after reading Survivalblog.com. It's gonna kill him to see the videos posted online! They made virtually the same filter only my number one did it without a pattern. If you have the time and enjoy the process of figuring out the design on your own, do it. If you like pictures and video, click the above sites then do it! Just decide what kind of filtration you are going to need and meet the need now while you can.
"mom" note: Buy back up filters. Several sets of back up filters. That which you don't use you can trade with those who have none.