I don't know if they require roll bars for drift days (practices), but if you are doing any track events, drifting @ horsethief, etc, you will need one.
Here's one that will work for basically all events requiring 4-pt roll bar:
DavisFAb S13 240sx Convertible Roll Bar Kit
^ The description & pics are old. The kit is now TIG welded (in picture it is MIG welded) & the base plates are cut better.
In my car - most recent pic I have:
