How long ago did you see Tarbell's grandson?
Was it in _______, Virginia? Do you remember his name?
If you know the answers to any of the above, you might be able to track him down. Maybe he lives in your area...who knows? He or his family, may have information or photos/papers, etc., that they may let you see. You could learn a different side of Tarbell from the family's point of view. If the grandson acted embarrassed, you don't know if it was because he didn't like attention to himself, or the family has a different,(positive/negative) view of him. Sometimes family conflicts get into the mix of what they remember. It may be they thought, "that magic stuff" was trivial, not caring about it. Some families are proud their grandparents made a living as a magician or mindreader, others are ashamed of it.
Consider this: If you contact them, and ask about Mr. Tarbell, you may be calling them, just before they were ready to clean out the basement and take "all of grandpa's magic junk" to trash man.