Nothing ventured, nothing gained. You just need to overcome the shyness and politely ask. The worst they can do is say "No". Not to state the obvious, but here are a few tips for road-trips:
Shower and be well groomed. Don't go in looking like a bum with 3 day old scruff on your face, wearing a hat to hide your bed-head.
Leave the sunglasses in your car. Don't worry, your retinas will survive the 30 second exposure from your car to the bank.
Smile and be polite with a simple greeting (Good Morning/Afternoon). Just don't storm the teller window and demand half dollars.
At this point, you can just state the obvious ("I'm looking to buy half dollars, and was hoping that you had some available.")
Or you can go in prepared with a ridiculous story, if they look like a tough cookie to crumble. ("I'm looking to buy some half dollars for my niece. She really is fascinated with the large coins and enjoys counting and separating them by year. It is the only common interest that her and I share, and I really love spending this special time with her.") Or something like that. Keep it simple. Don't bore them to death with some convoluted story, but a sweet, simple backstory helps to crumble the old ladies.
Always, thank them for their time, regardless.