I buy stevia from this website: http://www.steviva.com/store/
This is the product I always buy (after some research on the subject): SteviaSweet 95-60 - pure stevia extract powder - 1 KG
It comes to $138 and generally lasts me a year, less since my family has been eating healthier as well. If you google "Steviva coupon code" you can almost always find one that works and save from $25-$50 on this order. I just ordered it and paid $105 for it with a coupon code I found online.
To make the stevia for use, I boil 2 cups of water, turn the heat off and add 1 cup of stevia and mix until it's dissolved. I then keep it in a glass jar in the fridge and some in a glass dropper. One full squirt of the dropper is enough to sweeten a quart sized smoothie. For my chickory tea I add a dash of vanilla, 2 tsp of carob powder and 2 squirts of stevia. It's a very nice hot drink.