Monitoring free space in SQL Server data files
Not sure why you want to use performance counters for this when you can get it from a simple query. And in fact while you can get this information about log files from performance counters (Log File(s) Size (KB) / Log File(s) Used Size (KB)), there is no such counter for how much space is used in a data file.

;WITH f AS 
(
  SELECT name, size = size/128.0 FROM sys.database_files
),
s AS
(
  SELECT name, size, free = size-CONVERT(INT,FILEPROPERTY(name,'SpaceUsed'))/128.0
  FROM f
)
SELECT name, size, free, percent_free = free * 100.0 / size
FROM s;