This is not a low cost item as in dirt cheap. But look for someway to to treat or purify water. You can store a lot if you have room, but at some point you will need to replenish the supply. I went with multiple ways as time has passed.
1st I used to backpack a lot and got a good purifier (PUR) that I could carry, that will be great in a BO situation.
2nd we ended up getting a Berky purifier for the house for everyday and BI use. It'll do 3-4 gallon a day as a gravity feed.
3rd, I keep purification tablets in our GHB's for light weight and short term mobile use.

As time has went on I have picked up a second backpacking type (SteriPen) as a spare or for the wife should we have to split up for some reason. I will probably pick up a couple more of some type as time goes on. I also plan on setting up a pair of 5 gallon buckets with Berky type filters to use if we have more folks here. Plus multiple filters for spares.

On top of that keep a good supply of coffee filters, bandana's, or t-shirt material to use as a pre filter to keep your main filter as clean as possible.