Don't know, in my country it must be prescribed by a doctor, you can't just go and buy it. You won't get it on mass-market websites like Amazon, that's for sure. Look through this forum and other sources, I've seen the topic myself several times over the years (but didn't pay attention since I don't care where they get it from in the US).
Oh and by the way, get urine test stripes (cheap!) to make sure your kidneys work, it's a precondition for using chelators (a doctor doing such a general check will use those stripes too, more thorough methods are applied only when they already suspect something is wrong). I just had to think of that because DMPS works in the kidneys the most (which is a good thing).