Then you need to find another drive to extend it on.

30 GB is not really a small tempdb, but it not excessive either. All depending on your workload 30 GB may make sense, be too small, or be inflated by one single bad query.

You can find out which processes that are using a lot of tempdb from this DMV: sys.dm_db_task_space_usage

Do you have any databases that uses any form of snapshot isolation? You can see how much space the version store takes up in this DMV: sys.dm_db_file_space_usage.