Can I Search The Passwords Stored in my Keychain?
In the absence of any better suggestions ;)...

Unlock Safari > Preferences > Passwords.
Then select all entries (select one first, to bring the right area into focus, then select all entries by pressing the cmd+A keyboard keys), which will make all passwords visible (usually displayed as dots.  Earlier versions of Safari need the "Show passwords for selected websites" box ticked).
Now click on the "Password" tab, to sort them alphabetically.  Look for your password.

As bonus, all sites with the same password will also be displayed underneath each other.