It's possible you don't have one if you're using a SPFILE. If you were using an init.ora file the first place Oracle looks is the $ORACLE_HOME/dbs directory. The file it finds here is usually:

- A symbolic link to the $ORACLE_HOME/admin/$SID/pfile directory
- A file containing an IFILE reference to this directory.
- A SPFILE reference.

If you find the file in the DBS directory it will tell you where your file is if you have one.

Cheers