Where is the journal stored?

There is no fixed place for the journal, each filesystem stores it in a different place.

If you are expecting it to be a regular file, then your are mistaken, it's more like a part of the filesystem structure like the inode table. The funny point is that ext3/4 treats it exactly like a regular file. It is usually found in the inode number 8, but this is a kernel parameter that can be changed at compile time.

If you want to get technical, this article by Carlo Wood, written in March 2008, is a great read about the ext3 structure.

If you don't mind put journal in a place with words like partition table, inode or block and don't worry about it:

The journal is stored in a separate space of the partition, you cannot access it directly. You can use sudo debugfs /dev/sda# where the # symbol is the number of your partition, and then using logdump in debugfs to print it out. The output is confusing though.

Generally, as a regular user, you don't need to know about the journal. Using ext3/4 will be sufficient to prevent data loss (in most cases).

This link might help: http://www.linuxforums.org/forum/red-hat-fedora-linux/121074-ext3-journal.html