I have never quite understood why so many christians seem to think that "where do atheists get their morals from?" is such a good argument. This argument sounds just silly to me. It might be so because I live in an environment where something like 85% of people are secular, rather than living in a social bubble where 95% of people are (or claim to be) christian, where one can pretend that social order happens because 95% of people are christian and ignore the rest of the world.
The answer to the question is pretty simple: Humans are intelligent, thinking entities who can deduce and agree what's good and bad, and agree to the rules by which the society should be governed in order for it to prosper in the long run.
Ultimately, though, I believe that human moral values come from evolution. Many of those christians would consider such a claim preposterous, but it is, in fact, quite simple, really.
Evolution is a natural process that causes groups of living beings (in this particular case societal groups of humans) that are most fit to survival to get naturally selected over time over those who are less fit for survival.
Now, consider a hypothetical case of two groups of humans:
The first group lives with the moral principles that these christians envision atheist to have, in other words, they are completely immoral, each person only cares about himself, each person takes whatever he wants, they will abuse others for their own personal amusement and profit, and if someone doesn't like someone else, they will just kill them.
The second group lives by rules that the society has agreed on, based on the concept of basic human rights, and that every individual should cooperate with the rest of the society in order to make everybody's life better. Not because some imaginary "god" has told them so, but because they have rationally came to the conclusion that it's better for everybody if everybody cooperates.
Which one of these groups is more likely to survive in the long run? Which one of these groups will most likely be favored by natural selection?
Hence it's quite logical that groups of people who develop the societal cooperation mentality would have a significant edge in terms of survival over groups of people who are egoistic and selfish. Thus natural evolution explain the source of morality beautifully.