10" rear drums on 69 Dart

10" are correct for original disc brake and many if not all V-8 A-Bodies.
In the past few years, the only new ones available on an on again, off again basis have been the Centric 123-63044.
Right now, probably your best bet for new ones is to order some from Summit and hope they get shipped someday.
Centric Parts 123.63044 Centric C-Tek Standard Brake Drums | Summit Racing

Check with Amazon and Rock Auto frequently, too.