If you're consuming it as a supplement it's a lot cheaper (and safer) to just make it yourself. The process is basically: cut it up into thin slices, dry it out in the oven, grind it up.
If you're not taking it in supplement quantities, then I'm not sure I'd be tremendously worried about any other contamination...
If you're not taking it in supplement quantities, then I'm not sure I'd be tremendously worried about any other contamination...