I think the "Where do Atheists get their morals from" argument is extremely weak. Theists only use that when they have no real arguments left.
1) Where do Christians get their morals? Christians get their morals from the bible, which they believe was written by a supreme being. But God is supposed to be infallible, which means that he is not capable of making mistakes. If we can prove one part of the bible wrong, and we have proven many parts of it wrong, then it is proof that it wasn't written by a God. So if Christians morals didn't come from God, where did they come from? Like many scientists believe, our morals are simply encoded into our DNA. It is instinct.
2) And even if you won't accept that the Bible was not written by God, the fact of the matter is that Atheists do have morals, whether you believe that is possible without a belief in God or not. I am a decent person simply because I choose to be.
Many Christians can't understand how people can do good things without the fear of Gods wrath, or the promise of going to heaven. So tell me, who is the immoral one? The Atheist who is good because they want to be, or the theist who is only a decent person because of the fear of God or the promise of reward? Theists are the immoral ones.
And to the Theists who still can't understand how people can be good without God: if God exists, he can read your thoughts. He will know if you are only being good out of fear, or because you want to be rewarded when you die. If you know that you would be doing immoral things without the belief in God, he will know too. And you will be sent to hell just like the rest of the immoral people.