Where is the scripture that Jesus had a spirit body before he was born of Mary?
John 8:57-59
57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham?
58 Jesus said unto them, Verily, verily, I say unto you,
Before Abraham was, I am.
59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
Not just the LDS people but many other christian sects believe that Jesus was telling the Jews that he was the great I AM in verse 58. He was telling the Jews that he pre-existed his mortal life and was the Great I AM who revealed himself to Moses. For this reason the Jews took up stones to cast at him.
Exodus 3:13-14
13 And Moses said unto God, Behold, when I come unto the children of Israel, and shall say unto them, The God of your fathers hath sent me unto you; and they shall say to me, What is his name? what shall I say unto them?
14 And God said unto Moses,
I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel,
I AM hath sent me unto you.
This is the same being who in Exodus 33 revealed his spirit body unto Moses:
Exodus 33:17-23
17 And the Lord said unto Moses, I will do this thing also that thou hast spoken: for thou hast found grace in my sight, and I know thee by name.
18 And he said, I beseech thee, shew me thy glory.
19 And he said, I will make all my goodness pass before thee, and I will proclaim the name of the Lord before thee; and will be gracious to whom I will be gracious, and will shew mercy on whom I will shew mercy.
20 And he said, Thou canst not see my face: for there shall no man see me, and live.
21 And the Lord said, Behold, there is a place by me, and thou shalt stand upon a rock:
22 And it shall come to pass, while my glory passeth by, that I will put thee in a clift of the rock, and will cover thee with my hand while I pass by:
23 And I will take away mine hand, and thou shalt see my back parts: but my face shall not be seen.