food grade bags

So recently I started a project in which I am trying to create a water reservoir that is variable volume and in which the water has no contact with air.

The problem becomes that I need food grade bags with a combined capacity of 6 gallons with some sort of spigot attached. I tried using 1 gallon freezer bags, it is a nightmare to try to attach a fitting to them. I thought of using camp shower bags, but every bag I've found is made of either some un-named substance, or pvc. Thought about wine bags like the type used in boxes of wine but new with the wine in them they cost far too much for me to spend the money on them. "camel pack" would be suitable but again they run into too much money.

I was wondering if any of you fine people knew of any food grade bag that I could use/ and or fashion, without running into beaucoup bucks.

I promise to post the project in its entirety when I finish

 

LDPE and PP Drum Liners - US Plastic Corporation

These have round bottoms. HTH.

 

how about this?

Amazon.com: Coleman Expandable Water Carrier (5-Gallon): Sports & Outdoors

EDIT: Actually this is more what I meant
http://www.amazon.com/dp/B001DNZAJO?smid=A3URPP7JEEVM1B&tag=msnshop-sports-mp-20&linkCode=asn

 

this wouldn't work, I need it to work in a with slight or no pressure

EDIT: to clarify as the volume of the container above reached zero the negitive pressure in the container would increase. I need something that has almost no pressure