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.