Find below the query to find the free space of a tablespace:

SELECT DFQ.TABLESPACE_NAME "Tablespace Name"
,DFQ.TOTALSPACE "Total Size MB"
, (DFQ.TOTALSPACE - DSQ.TOTALUSEDSPACE) "Free Space MB"
,ROUND (100 * ( (DFQ.TOTALSPACE - DSQ.TOTALUSEDSPACE) / DFQ.TOTALSPACE)) || '%' "Free Space %"
FROM (SELECT TABLESPACE_NAME, ROUND (SUM (BYTES) / 1048576) TOTALSPACE
FROM DBA_DATA_FILES
GROUP BY TABLESPACE_NAME) DFQ
,(SELECT TABLESPACE_NAME, ROUND (SUM (BYTES) / (1024 * 1024)) TOTALUSEDSPACE
FROM DBA_SEGMENTS
GROUP BY TABLESPACE_NAME) DSQ
WHERE DFQ.TABLESPACE_NAME = DSQ.TABLESPACE_NAME(+);

Also, you can refer to this article to find a handful of detail about the tablespace:

Check Tablespace Usage in Oracle