DEPENDS... A year ago I did a exhaustive research and found these best options and budget... 

1) START NOW...

Best to buy 'distilled' water jugs at the store. Distilled is purified and bacterial free already and good for long-term storage. Spring and others are not. FEMA recommends 2 weeks of food and water. 1 gal of water per person per day is the average. Build up to at least 3 months, but start now.

2) PORTABLE WATER CONTAINERS

Everyone should have a few of 3 to 6 gal containers for hauling water (a set for clean water and dirty water for later filtering). Size depends on whatever YOU can carry, min 2, one for each arm. Water weighs per gallon: 8.33 pounds or 3.78 kilograms.

3) BULK WATER STORAGE

1-2 PEOPLE: Emergency Zone's Water Boxes (with 5 gal mylar bags). These are actually a re-purposing of restaurant food grade liquid containers used in the industry. My favorite option. $200 for 100 gallons of storage.
https://www.emergencyzone.com/collections/water-solutions/products/water-storagetreatment-set

FAMILY/GROUP: Things get really expensive, really quick, but the better options are the larger stackable 30 to 60 gallon containers and/or 55gal drums.

BEST LONG-TERM WATER TREATMENT (5+ years) is CL02 (Chlorine Dioxide) - not to be confused with Chlorine (CL1). They are two separate chemicals. Does the same but also eliminates slim build-up and taste problems. Bacteria can grow in slime so the regular Chlorine treatment is not 100%. I use the AquaMira products.

A MUST HAVE:  PERSONAL WATER FILTRATION units/straws/filters FOR EACH PERSON.
These are often used by backpackers, hikers, and campers. They take dirty water from a lake, creek, stream, or even pond or ditch and filter it where you can drink it safely.  They do not filter salt-water (there are actually other solutions for that). As the ultimate backup, you should have one for every child, teen, and adult that they carry with them in their pocket at all times during a disaster.  These are often very affordable (around $25 each, cheaper in quantity) and will most likely save that person's life in a disaster. Sawyer Mini is the one I use and highly recommend.  GROUP / FAMILY SIZE: There are larger filters available, some for very large groups [one example], but even if you have them, each person should still have their own personal water filter for ultimate safety because without water - you die. 
MORE INFO: Sawyer, Lifestraw, Aquamira.

ALTERNATIVES TO WATER STORAGE...

IF YOU HAVE LAND/HOME, even in an urban environment, consider talking to a Water Well Drilling company. Tell them want you want, electric with manual water hand pump option and use for drinking. They can help you with the rest of the details.

A few large water storage containers would also be advisable.

DAVE'S REPLY:

HEAT & SUNLIGHT:  Keeping water out of sunlight and even warm environments does significantly decrease the odds of bacterial growth.

TAP WATER:  Filling any type container with 'tap water' is okay, so long as you do a yearly flushing (empty, washing, refill). The warmer the water is stored, the more frequent you should flush it. I use a store-bought water filter in my kitchen and in filling water storage containers.

LONG-TERM STORAGE:  Requires chemical treatment to prevent bacterial, taste, and slime problems. The most effective chemical treatment against bacteria, slime, and odor for long term water storage (easily 5 years) is Chlorine Dioxide (CLO2). While Chlorine (CLO1) is commonly used to treat bacteria in water, like in pools and spas, it does not address taste and slime build up issues like Chlorine Dioxide (CL02) does. Since bacteria can grow in slime, Chlorine (CLO1) is not the recommended for 'long-term water storage.'

DISTILLED WATER:  Store-bought 'distilled water' jugs, the bacteria and impurities have already been boiled out of it/removed (as if you had done a long term chemical treatment). You can also buy and setup a water distiller in your kitchen to boil/purify your own water before storing.

DAVE'S REPLY:

Calcium Hypo-Chloride IS Chlorine in powder form. Chlorine is cheap and commonly available and is often used to treat water for bacteria.

The reason why many preppers tend to recommend Calcium Hypo-chloride is because it is the preferred long-term storage form of Chlorine (CL01) since it has a shelf stable life (in dry powder form) of 10 years, while Chlorine and Bottled Clorox in liquid form only last maybe a year. CL01 and Calcium Hypo-chloride can be much more dangerous to mix if you get things wrong. These chemicals are much more dangerous to store, are much more corrosive, and flammable upon interactions with other things. These chemicals are similar to the Chlorine Gas weapons used in World War 1. So, if you are educated in its controlled storage, mixing, and use then you are safe. If not, you can kill yourself or others.

Calcium Hypo-chloride breaks down to Chlorine (CL01). CLO2 (Chlorine Dioxide) and Chlorine (CL01) are two totally separate chemicals. BOTH kill bacteria in water in low concentrations and in high concentrations are used as sanitizing cleaners.

THE MAIN DIFFERENCE BETWEEN CL01 & CL02 is: CL02 is the only 'complete' long-term solution against slime build up and oder. Bacteria can grow in slime, so CL01 is not a complete solution against it. If you use city water, you've probably already been using CL02 in your tap water.

VIDEO: https://youtu.be/pc_ijTKp_lM

DAVE'S REPLY:

AquaMira is the chemical product I use... Chlorine Dioxide (CL02)

VIDEO:  https://youtu.be/em8v4Hj9kBM

WEBSITE:  https://aquamira.com/products/aquamira-water-treatment-2-oz-glass-bottles

DAVE'S REPLY:

Emergency Zone 5gal Water Boxes
It's the most cost-effective bulk long-term (5+ years) storage solution I've seen. The concept repurposes the Restaurant-grade, industry standard liquid food box containers to store just water instead. These are really solid and strong units, quick and easy to put together and fill. Each kit comes with everything you need including the water treatment (Chlorine Dioxide - CL02), water-faucet attachment and hose.

WEBSITE:  https://www.emergencyzone.com/collections/water-solutions/products/water-storagetreatment-set