No law. Just depends on the tellers. I have one branch ordering ten boxes of pennies at a time. At another branch the teller just don't want to give one full box.
banks refuse to sell me pennies, they told me to go to another bank... went to that other bank, they also refused to sell me pennies, sent me to ANOTHER BANK! That bank also said no.
Moral of the story, banks do not like to sell you pennies. ask for half cents instead.