Wow - yes Aspartame is dangerous, it's good you're trying to eliminate it!
Stevia can be bought at some grocery stores and most health food stores. It can be greenish or white, powder or liquid. You'll have to experiment with the right amount to add to your drinks and food.
At the top of the page it says "Foods" and you can read the foods that heal, it mentions the good sweeteners, there are others, in fact it's good to just browse the lists of foods that heal and foods that kill :-)