Steps to find Outlook Web backup file:
Check the default locations to verifying the existing location is always recommended. There may be some pst-files which you might have forgotten to connect with but still want to keep it anyway. The default locations of backup files are:
C:\Users\%username%\AppData\Local\Microsoft\Outlook\
C:\Users\%username%\Documents\Outlook Files\
You may have more than one .pst file you are using. Note the path and file name of each of these files. The name of any .pst file (other than the default - Outlook.pst) will be of same name within your folder hierarchy.