Where did God get the perfect sacrifice to redeem mankind?

Good question! Because if all that was needed to save mankind from their sins was a perfect man, it would have been a simple solution for God to create one. But that would have violated God’s established principle found at Psalm 49:7:

"No MAN can by any means redeem his brother Or give to God a ransom for him—" The Psalmist goes on to say that the ransom for a life is too costly for a man to pay, and then he declares, in faith, "But God will redeem my life from the grave; he will surely take me to himself." The Psalmist understood what it would take to redeem a human life from the grave - something infinitely GREATER than the blood of man would be required in order to redeem ALL of mankind from ALL of their sins throughout ALL time - only God could provide sufficient ransom!

And he was right! That’s why, when the time was right, God sent His only Son - born of woman and conceived by Holy Spirit - to redeem mankind.

"Knowing that you were not redeemed with perishable things like silver or gold from your futile way of life inherited from your forefathers, but with precious blood, as of a lamb unblemished and spotless, the blood of Christ." (1 Peter 1:18-19)

Acts 20:28: "Be on guard for yourselves and for all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood."

1 Timothy 3:16: "And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory."

Isaiah 63:8-9: For He [YHWH] said, "Surely, they are My people, Sons who will not deal falsely." So He became their Savior. In all their affliction He was afflicted, And the angel of His presence saved them; In His love and in His mercy He redeemed them"