First off, don't be shy about it. Remember you have the account there, so you are the customer. The worst case scenario is they say no, and you walk out no worse for the wear.
Whenever I go into a new "order" bank, I walk up to the next available teller and ask them first if they have any Half Dollars. Nine times out of ten they say no. Then I ask about ordering Half Dollars. Usually they have to ask the Vault or Lead Teller if they can do that. I then strike up a conversation with the Lead Teller. HONESTY is ALWAYS the best policy. If they should ask why I want so many coins, I always reply that I am a collector. I assure them that I will be there the same day the coins arrive to pick up my order, and that I will not under any circumstances bring the coins back to them for deposit. In my experience the biggest concerns all Lead Tellers have about ordering Half Dollars is that they won't be picked up or they will be searched and brought right back. As long as you can assuage those fears you should be in the clear.
Confidence, Honesty, and Promptness is all it takes.
Good Luck and Happy Hunting!!