This tank is 160 gallons.
And this is 100 gallons (Apologizes for the reuse of the photos - I may update these when I find fresh photos in my files. Today is about content...)
I'm going to start by talking about the tanks themselves. These are livestock watering troughs, in zinc-coated steel, available in many shapes and sizes. My 100 gallon tank is made for sheep and goats, and the taller 160 for horses and cattle.
If you are lucky enough to live in a rural area, these are available at your local feedstore. If you don't see them, ask, as they are likely to be stowed out in back somewhere, waiting to be looked for. They take up a lot of 'shelf space.'
A search for 'stock tanks' should get you what you are looking for.* Here's the posting at Tractor Supply, a store we had around the corner from our place in Michigan.
Other styles of tanks are available, such as the tough black plastic Rubbermaid models. If you are in a climate with cooler nights in the summer, these may be a good investment. I might have bought one in Michigan, and softened its aesthetic by surrounding it with potted plants - but here in the desert all that black would just cook the poor pond. Phoenix is not a place for solar absorptive black.
Once you get it home, put it where you think it will stay. Keep in mind that many flowering pond plants require 4-6 hours of direct sun to bloom, lilies being particular sun-worshipers. Check to see if you are in the path of any regular leaf drop - I've a neighbor's eucalyptus that is the bane of my tall ponds existence. I pull leaves out of the water every day, and eucalyptus are NOT friendly to other forms of plant life.
Next, check to see if your base is level - any distortion of the bottom of the pond will shorten the life span of the tank. Pick up a few inexpensive bags of builder's sand and use this to create a bed. (Those of you who have erected free standing swimming pools will recognize this process.)
I had one tank that sat square on the concrete, and didn't use any sand beneath it. The other I needed sand - we just dumped the bag and then shifted the empty tank back and forth across this surface until it was stable. I did not bother to level my tanks, and as they are on a slab concrete patio, and both show the slope in their water lines. I plant my tanks heavily and this inconsistency does not bother me. Your mileage may vary. If you are going for those clean lines of modernism, take this time to pull out the level gauge and make sure the sand creates the proper footing for your final product.
*A note to my British and European visitors - If you know of a source for these tanks on your side of the pond, please speak up! I had a go looking for a retailer over there with no luck. A blogger in England would like to find them, as they fit her design scheme! Many thanks in advance!