I have built small rainwater recovery systems before, using food grade plastic 55 gallon barrels. My henhouse has such a system and it provides them with unlimited water that I do not have to haul or pipe out there, or buy from the City. Here's a look at that system:
The coop roof slopes back, and all of the water is caught in a gutter and diverted into the barrel until it is full, then into the downspout. A standard poultry watering dish finished the project. Please not the substantial 4x4 supports required to hold that 400 pound barrel up.
For the new system I needed a lot more than 55 gallons.
I shopped around online and came across "Plastic Mart"...
http://www.plastic-mart.com/class.php?item=3698
That's the page I used to order my tank. In short, they are a Nationwide broker for plastic tanks, and they arrange to ship these tanks anywhere in the Country from the nearest manufacturer. As it turned out I have a manufacturer 10 minutes from my home, so I got a 300 gallon tank wholesale online and picked it up myself.
The only problem with a tank that size is the ultimate weight when filled. Water weighs 8 pounds per gallon, so a 300 gallon tank will hold 2400 pounds of water. In addition, you need to have the tank high enough to create a "head" in order to move the water around without a pump.
The site is between the end of my house and the fence, and is about 7 1/2 feet wide. The plan is to put in a retaining wall down the middle with a paved path to the left and a raised bed to the right. In the foreground is where the tank will eventually go.
I started by shoveling all of the topsoil out of the left half and onto the right half where the bed will be, and laid down a strip of landscape barrier cloth. Then I moved about three yards of gravel in and buried the cloth.
The tank is 43 inches in diameter, so I excavated a 6' square foundation about 8" deep, and backfilled it with gravel up to the level of the filled pathway. I rented a plate compacter and packed all of the gravel down thoroughly. I checked it for level, adjusted it and packed the foundation again.
I used dense pre-formed concrete landscaping stones, and placed 11 of them in a circle over a thin bed of sand, and 10 more on top of those. I used cargo straps to cinch the blocks together and keep them from moving until I could get weight on them.
It's important to note that the center of the blocks has to be filled and solidly. I used rocks and rubble to take up some space, then filled the whole volume with gravel. I packed it with a steel rod and a hand tamper, then added several bags of coarse sand, and tamped it in too, until the whole pad was completely filled. If it does not support the tank, the bottom will fail.
Then it was a matter of installing pavers around the foundation and filling the gaps with sand to ensure the bottom will never move. At this point I was ready to set the tank.
Another important feature to consider is mosquito-proofing the tank. I made a collar to fit the manhole and installed a cut-down plastic bucket in it. Inside the bucket I stretch a couple of inexpensive nylon paint filters and hold them in place with the bucket lid. Then its a simple matter of cutting an appropriate hole in the lid to fit the downspout, and you're there.
That's what it looks like this Spring. I set the tank on a Friday afternoon and got it roughly plumbed enough to hold water, and overnight in a moderate rain the tank caught 175 galloons! By Sunday morning it was full.
I am using 1" black plastic pipe and drip irrigation fittings to irrigate the raised bed. When the tank is filled it overflows into the drip irrigation emitters.
Again, this is not the only way to do something like this but it is certainly cost effective. Home Depot and Lowes both sell a similar 50 gallon plastic "rain barrel" for $100, and you would need 6 of them to match the capacity of this tank. This is simpler and a lot cheaper, not to mention a 100% renewable and sustainable way to provide no only constant irrigation for growing some of our own food, but an important source of emergency drinking water that is ready for filtering and purifying should TSHTF...