The Dallas DSO covered the states of Texas. Oklahoma, Arkansas and Louisiana. As the other poster said there were thousands of dealerships that could have sold your car. There is not any definitive list of dealers. Kevin Marti owns perhaps the best list, he had to reverse engineer his list by collecting dealer numbers from enthusiasts who keep registries (like me for example) and from so - called "Eminger Reports" and from any other sorces he could find. Needless to say Kevin will not share what information that he has.
Also many cars were sold through dealer transfers that might go to a different DSO. So no, there is not any way to determine where a particular car was sold using the DSO number as a source.
Is there a list of dealerships and which DSOs they were a part of in the mid 60s? I am trying to figure out which dealerships my 64.5 could have came from with a Dallas DSO.