Managing Gaps in Numbering Sequences

I'm creating a simple database to keep track of a DVD collection, where the media are going to be removed from original packaging and stored in small paper sleeves. Each disc will get a record in the Media Table and the client wants to simply assign each Disc a unique sequential Location #.  So: D-0001 D-0002 D-0003 etc.

We would like to have an "Assign Location #" button that looks up the existing numbers and offers up the next available number in the sequence.  To complicate matters a bit, we also want to plan for filling gaps that are created when obsolete DVD's are removed from the collection due to damage, obsolescence, etc.  So say the highest number in the existing sequence were D-0333, but there are currently no DVD's assigned to D-0092, and D-0300, we'd want the user to be able to choose between the missing numbers or D-0334 (it's preferable for related Discs to be next to each other in the drawer).

So, I'm thinking maybe rather than manipulating some kind of Serial $ field in the Media Table I should be making a related Location Number table with a one-to-one relationship so that these Location numbers are an Entity.  I'm vaguely thinking I could use a filtered portal to display Location slots that currently have no value in the key field. sorted by Location #. Maybe have the Location selector be a little window that pops up when you want to assign a number?

Is that the right approach? Is there a better one?  

One follow-up question is if I take this approach, should I just create a large enough number of Location #'s that will ensure they never reach the final number . . . or are there unfortunate consequences for that I'm not anticipating?