I am trying to find/locate iMessages from a certain day back at the beginning of October. Is this possible? Scrolling back on the history on my iMac in Messages goes back to 10/19/15 but no further. On iPhone it is more recent than that.
If it was archived on your Mac, you can find it by date.
Open the relevant dated folder & any conversations for that day will be listed by the contact names.
Double-clicking will open it in Messages.
The last I tried this was OS X Mountain Lion and iOS 6.x on a jailbroken device. There is a messages database located on each device. You can Google for the exact location in your OS versions.
I know these databases used to be SQLite, and you could browse the database using SQLite commands and see all info stored in the entire database.
Be sure to make a backup first, and ideally, do all your investigation in the backup database.
There also used to be a Cydia app that would allow you to search all of your iMessages by date/contact/etc., but the name escapes me at the moment.
This may not be the greatest answer, but it's all I've got, and at least it's something!