I have the 'Creator' level package for my website and it says there is 500GB of media storage available. Wondering where do I go in my admin view to see how much media storage has already been used? Also is there any way to purchase additional media storage a la carte? 

This would be found in the Client Area (not the ACP) -> Manage Purchases -> click your package -> scroll down to storage usage.

At this time, we do not provide storage a la carte but if you have a concern, we can put you in touch with our sales team.