The Berky is the 'gold standard' of gravity filters, if you can afford one Frugal's
is a great place to get it. Stainless steel isn't cheap, and their products are top notch.
There are other options that are a lot cheaper; new companies with similar specs, DIY containers using aftermarket replacement filters, boiling, bleach, UV, honestly there are a lot of ways to make safe water, my parents got suckered into a expensive water distiller in the 80's, it does work for water, might even work for other 'liquids'.... I might have to check it out.
Honestly, safe water is a matter of circumstance and balance, you can eliminate organic contaminants with filters, uv, or bleach. Distillation removes almost everything but leaves the water naked of essential minerals and it tastes wrong to my palate. I add a pinch of sea salt to distilled water and agitate it by pouring it between two containers 10 times or so.
Pure water is two hydrogen molecules combined with one oxygen molecule, but most of us really wouldn't recognize it from what we're used to drinking and even if we were, our bodies wouldn't be satisfied without the other trace minerals in a long term situation.
Ocean salt water isn't as bad as many survival texts state, from what I've read, mixed 1/7 to 1/9 with distilled water it's beneficial adding back in the lost minerals. I've mixed it this way and it tastes ok'ish, I haven't preformed a chemical analysis.
So.. How do I prepare? I use 1 gallon milk jugs with 8-10 drops of bleach replaced every year, I store about 20 gallons this way; it depends on the space on the shelf. I rotate a gallon of bleach with our laundry every year, it's a backup for about 1000 gallons, give or take. I can always boil water and I have a Katadyn water filter for short term needs.
Water off your roof through the downspouts is very viable if there hasn't been a chemical or nuclear event, it's easy to collect and requires very little treatment. Boiling or bleach is easily enough.
When it comes to boiling water the stated goal is to boil the water at 212 degrees, but actually the goal is pasteurization and not boiling. The idea is to hit 162 degrees for 20 seconds, this is all that is required to kill known bacterial bad guys.
Just a few notes from the field..