Aloha!
By "mainstream" do you mean nationally available? I would have to vote for certain cabinet cards and probably anything 19th century that would fit into mainstream. I have found that most all 19th century regional issues are tough, but I guess thats not mainstream...hmmmm how about some of the tougher sporting news issued cards that are in high demand. Then there are the tough series of cards in a few prewar gum issues too, I guess one could go on and on. Peace, Dave.
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