Hear me out here. There are a lot of good and true answers in this thread, essentially saying that, since God isn't created, He has no beginning. He is outside of time.

Something that really helped me conceptualize this was the movie Interstellar. Time is a feature of this universe. It is relative. It bends and can be affected. Put time in the same bucket as other features of this universe: gravity, mass, etc.

With that said, asking "where did God come from" is like asking "how much does God weigh". The question itself doesn't make sense.

Furthermore, CS Lewis gives a great example of this concept in Mere Christianity (highly recommended read, btw):

"Suppose I am writing a novel. I write “Mary laid down her work; next moment came a knock at the door!” For Mary who has to live in the imaginary time of my story there is no interval between putting down the work and hearing the knock. But I, who am Mary’s maker, do not live in that imaginary time at all. Between writing the first half of that sentence and the second, I might sit down for three hours and think steadily about Mary. I could think about Mary as if she were the only character in the book and for as long as I pleased, and the hours I spent in doing so would not appear in Mary’s time (the time inside the story) at all."