I thought this question had been adequately dealt with years ago, but it seems to have made a reappearance, so I thought I would address it.
If you ask, "Where did God come from?" you are incorrectly including in your question the idea that God is a part of time, rather than the creator of time.
It does not make sense to ask what came "before" God because words like "before" and "earlier" and "previous" and so forth are all about time, but God made time and therefore is separate from time. And if something is separate from time, then there is no "before," and asking what is before God is as silly as asking, "What is before blue?" It is not a time question so you can't get a time answer.
Think of time as a line you draw on a sheet of paper. If someone asked what comes before you on that line, the question wouldn't really make sense because you are not part of the line. It's the same with God.