You could keep watching until they come up for sale, and write the sellers before they are sold offering to buy a copy of the manual.
They would need to go somewhere like Staples and wait for a copy to be made, so it’s their time you are compensating for.
Here’s one now
https://www.usaudiomart.com/details/650015063-threshold-t3i/
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usaudiomart and others have wanted sections, you might get lucky. ask for original OR copy.
https://www.usaudiomart.com/classifieds/all/?filter=WANTED