If I was going to use Stevia as a sweetener then I'd grow my own herb at home and dry the leaves to use in cooking, tea etc that way I know exactly what I'm consuming. I believe the Stevia herb itself is safe but I have no trust in the manufacturing process involved by these large sweetener companies and I do not trust their claims that their product is safe. This is just my personal view based on what I have read and my belief that large companies like this will put profit before customer health.
This might be interesting, taken from Wikipedia:
"Rebiana is the trade name for high-purity rebaudioside A[1], a Steviol glycoside which when used as a non-nutritive sweetener is 200 times sweeter than sugar.[2] It is the primary source of sweetness in the Truvia sweetener brand. According to the Truvia website, Rebiana is derived from Stevia leaves by steeping them in water.[3] The Guardian newspaper provides an alternative view of this claim - "In fact, Coca Cola's patent describes a 42-step procedure to derive Rebiana from the stevia leaves, using such country chemicals as acetone, methanol, acetonitrile, isopropanol, tert-butanol and "mixtures thereof". Whether Truvia can truly be classed "natural" seems, at the least, semantic.""
Someone earlier asked where to get seeds from. There are some available on Ebay, apparently it is very difficult to germinate the seeds so might be better to buy the plant from a garden centre (some online garden nurseries are selling the plant, google).