SYSDBA is not a user, it is a system privilege. When you connect as
/ you are connecting to the
SYS user. You don't need a password when you connect as you are - as long as it's a local connection from an OS account in the DBA group.
You haven't said what happens when you try to connect, or why you think you need a password; possibly that is that failing (in which case giving the error message would be helpful) and giving you a SQL*Plus login prompt, maybe indicating you aren't in that group.
Passwords are stored within the DB, the exact location depends on version; but they're stored hashed (of course) so you're not going to be able to get the actual passwords in plaintext (without a lot of time and effort anyway).