Your pfile will be present in $ORACLE_HOME\database directory and you will get your SID attached to file as follows init<sid>.ora
The problem with just copying the init file is, if changes were made to the spfile and a new init file wasn't created, then these changes won't be reflected in the init file. So it would be better to create an init file before taking a backup.
You can create a pfile from spfile like, (Assuming that you have the necessary privileges)
create spfile from pfile; --this will create the file in $ORACLE_HOME\database directory.
create spfile from pfile='location' ;