Shoe and boot lasts sell for a little as five to ten dollars at antique shops and salvage stores. They can be made of wood or cast iron. After World War II, they were made of plastic. There are many around as there were many shoe factories in the North East USA. Hardwoods like ash were used to make them, so they last forever.
When you hunt for an answer to a question, it's best to be as specific as possible. In a search, mention the material the last is made of if it's wood or cast iron. When you suggest an age, try to estimate. You can't measure age by mentioning your relatives. One person's grandfather could have been born in 1960, another's grandfather could have been born in 1902. Age makes a difference.
When you say "set" do you mean a pair or a whole bunch that runs the gamut of sizes?
If your mother had them appraised there should be paperwork somewhere. A bona fide appraisal is written down. If it was word of mouth it was not done by a professional.