Purchasing Water Storage Containers

WaterBob

This first link describes a WaterBob, which is relatively inexpensive (around $24 on Amazon) and you can place in the bathtub and fill up with water during an emergency: http://survivalcache.com/emergency-water-storage/
However, if you don't have enough advance warning, this is not a good option, because if the public water system goes down before you can fill up the WaterBob, there goes the water.

Gallon Containers

The next two links show a 100 Gallon plastic container, but at this website you're not limited to a 100 Gallon amount - there's a good range from 45 Gallons to 250 Gallons: http://www.plastic-mart.com/product/6127/100-gallon-vertical-water-storage-tank-tc2364ic-2?gclid=COLyi9Sw6rICFSemPAodiFoAbA
http://www.plastic-mart.com/category/385/chemtainer-water-tanks

I personally like this option if you have room in your house, because you can fill it up before an emergency, adding a bit of bleach to purify the water (http://iwasprepared.blogspot.com/2012/09/water-storage.html).
It also has the added benefit that you could use it as a makeshift water filter if you use all of the water from the container (http://iwasprepared.blogspot.com/2012/09/build-your-own-water-filter.html).

Where to Buy

As far as locations of where to buy containers like these, I don't know of many places that sell large bulk containers - the closest I could find at Costco was 55 Gallon containers (http://www.costco.com/water-barrels-storage-filtration.html) - so that could be an option to get several of those containers.  However, the price is somewhat more expensive than http://www.plastic-mart.com.
The water containers at Costco are through Shelf Reliance, which I think you can sometimes get a good deal from Shelf Reliance Independent Consultants.  So if you know somebody that is an Independent Consultant you could try to contact them and see if that's a possibility.

I will also try to do some more research, and if I find more information, you can see that at our Provident Living blog - http://iwasprepared.blogspot.com