How do I find used/free space in a TEMPORARY tablespace?

Unlike normal tablespaces, true temporary tablespace information is not listed in DBA_FREE_SPACE. Instead use the V$TEMP_SPACE_HEADER view:

SELECT tablespace_name, SUM(bytes_used), SUM(bytes_free)
FROM   V$temp_space_header
GROUP  BY tablespace_name;

To report true free space within the used portion of the TEMPFILE:

SELECT   A.tablespace_name tablespace, D.mb_total,
         SUM (A.used_blocks * D.block_size) / 1024 / 1024 mb_used,
         D.mb_total - SUM (A.used_blocks * D.block_size) / 1024 / 1024 mb_free
FROM     v$sort_segment A,
         (
         SELECT   B.name, C.block_size, SUM (C.bytes) / 1024 / 1024 mb_total
         FROM     v$tablespace B, v$tempfile C
         WHERE    B.ts#= C.ts#
         GROUP BY B.name, C.block_size
         ) D
WHERE    A.tablespace_name = D.name
GROUP by A.tablespace_name, D.mb_total;