I started swapping bills with the bank when I was about 10 years old. Old notes usually get set aside for mutilation, so they are already set aside which is nice. By all means, go to you local banks, and ask if they have anything interesting. Be polite, and respectful of their time (ie: do your best to never hold back a lineup).
Once they get to know you,, and depending on the actual type of teller you are dealing with, they may call you when stuff comes in. It is about establishing rapport, and maintaining it too.
Good luck in your hunts.
Hudson