Where do dead in Christ rise from if they're with Jesus?

Just as those that rose with Christ when he was resurrected, those that rise first after the second coming will rise from their graves:

52 And the graves were opened; and many bodies of the saints which slept arose,

53 And came out of the graves after his resurrection, and went into the holy city, and appeared unto many. (Matt 27:52-53)

Their Spirits are with Christ, but their bodies remain on earth until they are resurrected.

This is a very common mistake. If you only read verses 16 and 17:

For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. [1 Thessalonians 4:16, 17]

It sounds like the dead in Christ are only rising, perhaps seconds before the rest of us. But you need to start the passage a few verese earlier at verse 13. (Many Bibles have a paragraph marker there). You will find your answer in verse 14. (in bold letters below)

But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope. For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him. For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent (preceed) them which are asleep.[1 Thessalonians 4:13-15]

So if the dead are already with Jesus where are the dead in Christ rising from?

Those who died in faith in the Old Testament are to be raised at the second coming of Christ.

Daniel 12:2 And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.

Hebrews 11:40 God having provided some better thing for us, that they without us should not be made perfect.

Revelation 20:6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.

Those who become Christians after Christ get their new life right away.

John 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.

Because Christians have this new life in Christ while we are still alive, when the body dies, it remains here on earth, but the soul is with Jesus.

2 Corinthians 5:8 We are confident, I say, and willing rather to be absent from the body, and to be present with the Lord.

This is how when Jesus returns for the rapture and brings with him the souls of dead Christians so that they are reunited with their resurrection bodies and we who are alive will be joined with them in bodies that are changed rather than resurrected.

1 Thessalonians 4:14-17 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him. For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep. For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.

1 Corinthians 15:51-52 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed, In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.