Where can I get a water-like canteen which you can pour adulterated water into and have it come out purified for drinking?

The Seychelle filtration systems seem to meet your needs. I've never tried one myself and can't attest to their effectiveness, but the description reads well. They come in plastic water bottle type and canteen type. I've found them advertised on several sites, but only one site I visited had simple order forms and information.

Seychelle Water Purification Systems