The magnesium malate, l-selenomethionine (usually labeled just "selenium", check the ingredients - selenium methionate is the same thing), ascorbic acid and ascorbyl palmitate should be available at a health food store which sells supplements, like The Vitamin Shoppe, Sprouts or Whole Foods (from lowest to highest price). The ascorbic acid is really cheap at Trader Joe's if you have one near you, and is labeled "Vitamin C crystals". Vitamin Shoppe sells online (http://thevitaminshoppe.com), and their prices, especially on their house brand, are hard to beat.
The nigari was a little harder to find. Like the ascorbyl palmitate and magnesium malate, it's more my own preference than anything specific to the protocol recommended here. Some sources say that magnesium chloride is a good form of magnesium to take, and I figure if it's one of the major forms of magnesium found in seawater, my cells will like it. After researching pricing, I went with herbalhut.com, and chose the unrefined variety, because refined is likely to be only magnesium chloride, minus the other trace minerals from seawater. BTW, Epsom Salt (magnesium sulfate) is also found naturally in seawater.