Find / Display Next Available Number?

I am creating a form for users to enter data. Each entry needs to have a "Event ID" associated with it and these Event IDs may be used more than once but will refer to the same Event (if it was a single entry with no duplicates, I would just use the rows primary key). How to create a query that looks at a list of existing numbers (Events already in the table) and returns the next/an unused number? I would like to create a button that says "Next available Event ID" and provides the user with a number.