There are still some sources that may have them available but will take a little Internet time to find them.
Another possibility, if 100% correct is not an issue, is to use a 1966 Emergency Flasher Switch and Wiring.
*Discard the Black Mounting Bracket but keep the nut so you can mount the Switch under the Dash as mentioned in a previous response by C5ZZ.
*Paint the Chrome Switch Knob Black for the correct appearance or leave it Chrome if it's not an issue.
*The '66 Wiring is longer as it has to reach the Glove Compartment but the Connectors are the same; just have to neatly wire tie the excess Wiring.
*The '65 Switch has a Bracket for the Flasher attached to it, the '66 does not. You may be able to use the same Bracket type as is used for the turn
Signal Flasher. Reproductions of that Bracket are readily available and often show up on eBay as well.
Great thread!
Ray