I don't know about the seeds for wheat grass but I bought a small pot of wheat grass at PetCo to plant for the cats to nibble on.

Bulk food, health food or specialized sprouting stores.

Think I've seen something somewhere suggesting that Jimson Weed(Datura stramonium) can be helpful in taking up heavy metals, such as arsenic & cadmium, from soils. 'course, that plant, in and of itself, poses some toxic problems of its own.

Vetiver grass is used in the tropics to remove pollutants from the soil. It will not grow in your zone.
Ask at any health food store for wheat berries. They should be stored in the fridge. Sprout three tablespoons of the berries for two loaves of bread. Easy and delicious.

raintreenursery.com had some toxin (arsenic among others) removing plants listed last year...They would be annual to be disposed of in your zone...

I did a google search at one point, and was suprised at the names of plants that popped up. Many common plants remove all sorts of chemicals one would not expect.

Another to look up for concentrating metals (Lead, I think) in its tissues is the sumflower grown for seed (oil snack, and bird feed). But as Lucky points out, if you use plants to help decontaminate an area, you then have to find a permissable place to dispose of those plants.

you can get seeds called wheatberries at health food stores or the bulk section at the grocery store. They are very cheap. What you do is soak them in water for a day then throw them in a tray of dirt, they'll sprout up in another day or so, and be ready to cut in about a week to be juiced. A nice sunny spot is recommended. I wouldn't grow it outside because of weeds and animals though, i did that and had slugs crawling in it and dogs urinating on it, killing it.