Annoyingly, I wrote the following once and it didn't transmit, so once again;
The reason you can't find half inch 10s is because they don't exist. The best I can suggest is that you get some longer 10's with the thread running right up to the head, and saw them, off to half an inch. You would then have to make pilot holes in the shelf with a bradawl and then screw a longer screw into each hole to the depth of half an inch to form a 'thread' in the fibres of the wood, remove it and screw in your shortened screws.