mikeg103,
Searching the web (Craigslist, FB Marketplace, eBay, etc) is probably your first go-to to see if you can locate one.
You can always post an ad right here on the Bolt
Truck Parts Wanted.
No idea what you may need or want to pay. Over that past couple of years I have seen 3 speed OD transmissions for '60's trucks go anywhere from $250-$700 depending on condition and completeness (having all the solenoids, controls, wiring etc.).
I will keep an eye out on my various web roaming sessions and let you know if I spot anything that may fill the bill.
Dan
