You have to contact Olympus' parts department. Say that you want to purchase the front element.
This is very strange though. Front elements don't just "pop off." They are usually (always) held in place by a rotating locking ring. This locking ring either somehow unscrewed itself it was destroyed, so you'll have to buy the locking ring as well as the front element, as well as try and make the proprietary spanner tool that is used to tighten the locking ring. But locking rings don't just pop off either. Perhaps the threads have been stripped. Perhaps the lens barrel itself is damaged, resulting in the locking ring popping out along with the front element.