I wouldn't worry about purity, not much need to be added to stevia, and you always want to dilute it, to make it easier to measure.

I think the brand "NOW" is very popular, they have both powdered and liqudi forms. The powder is very concentrated, and some use the powder to make their own liquid stevia, combining 1/4 teaspoon with 2 Tbs water to fill a small bottle, it's very tricky to dilute, and must be refrigirated and will stay good for a couple of weeks. The first link, "Paul on sweetners" explains how to do this. I've been using a tiny bottle for a year, and it's half full.

Wa'il