Why can't I get a pack of fifty, new $1 bills from the bank?

I will e hosting an event that will have attendees from all over the country. I planned to hand out a WheresGeorge.com dollar bill to each attendee as party favors so we could have fun watching where the money goes.
When, I went to the bank to get the bills today I was told there is a Federal rule that says new money cannot be handed out. What??? Is this true?
Why not? Does this limitation enhance “national security”? What are they afraid of? I really don’t get it. Do you?
Do you have any idea how I can get new bills? Was the bank teller just making this story up?