If you do not have the actual bolt anymore, not even a hardware store would know where to begin. If you don't want to bug your LBS, this is precisely the kind of assignment a Bike Co-op is set up to accomplish. You bring in your seatpost and test fit a few bolts until you find one that works. While you wait, on the house. Don't be shocked if the actual solution involves a new seatpost. Nominal charge. What are you waiting for?