How do I get a transcript of iMessage for legal reasons?

Are these messages still on your phone? If so, you can use an application such as PhoneView (Mac) or TouchCopy (Mac or Windows) to create a pdf or text file of the conversation which will include date/time stamps. However, before doing anything, you should consult your attorney.

If both you and the person you sent the iMessage to have deleted it, your options are somewhat limited. iMessages are end-to-end encrypted, which means even apple can't see them.

If you have iCloud or iTunes backups, then you can use them to restore your device to an earlier state before those messages were deleted.

You might be able to use third-party data recovery services, however these are not owned/managed by apple and there is no guarantee that they will recover the data you're looking for (if the iPhone has been erased, this is not a practical option).