We like our tea on the sweet side here, and drink it all day in the cold weather. I get Trader Joe's 100% organic stevia extract and have 2 of those tiny little scoops in each cup of tea...and probably 5-6 cups of tea a day. Any reason to lighten up?

As for the stevia itself it would be hard to overdo.  Stevia does contain saponins that can have a laxative effect, but it would take a lot of stevia to do that.

I have seen the Trader Joe's brand though and it is cut with lactose, which some people cannot tolerate.

Also, what is an inexpensive source for stevia?

Buying the pure 90 or 95% extract uncut is the best way to get it.  It will seem expensive when you see the ounce or pound price.  But you have to keep in mind that you use only tiny amounts of it and you are not paying for the shipping weight of the cutting additives in most stevia products such as FOS, erythritol, lactose, etc.

Most companies offering the pure extract sell it for about $11.00/ounce.  A local store has it for about $3 something an ounce.  Beyond that I have not looked much at what is available commercially because I buy my bulk from raw material suppliers several kilos at a time.  Even that is not cheap as far as kilo prices go, but again you use so little that it lasts forever.  I think the last time I bought it was something like 5 years ago and I still have some left.

Are you looking for the pure extract or one cut so you can spread it, such as over cereal?  If you are also going to bake with it then you would be better off with the extract cut with FOS so the FOS will help hold moisture in the baked goods.  Stevia alone does not do this.

Do I reduce the bitter effect of my jiaogulan tea by putting stevia in it?

No.  The sweet and bitter receptors are on different parts of the tongue.

 Just be on the lookout for neurological disturbances. I used stevia freely for years until my feet started tingling and then hurting when I walked. I stopped stevia cold and it went away.

I don't trust white powder.