any aoto parts store that still has the old school Dorman cabinets should have them; forget about the Checkers/OReillys, Useless zone-type places. an old Mom and Pop type or a regular traditional franchise like Napa Car Quest, Bumper to Bumper, Federated etc would be your best bet.
What you need are known as distorted thread lock nuts or crimped nuts. You can get these from some hardware places, Fastenal and McMaster-Carr. If I remember right, the factory used what McMaster calls a oval lock with conical top, much like the nuts used to hold a master cylinder in place. This link will get you in the ballpark:
On my '84 w-350 crew cab I still have the OG nuts with lock plates and on the rear I have the nylon lock nuts. They are one time use only at 40$ each. I'm coing to switch all four to Stage 8 locking nuts. Summit carries the Stage 8 kits for Dana 60's.
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