Well, I've had the exact same problem yesterday. I just couldn't find the damn things. After some surfing I found out that it may happen that due to a glitch the spriggans just won't appear when you get the journal entry.
You can fix this in two ways, both using the console.
The first is to manually add them, that way you can kill them as usual. You just create the ones you're supposed to kill, and then you send 'em off and get the journal entry.
Here's the command
placeatpc "BM_spriggan_co1" 1,1,1
placeatpc "BM_spriggan_co2" 1,1,1
placeatpc "BM_spriggan_co3" 1,1,1
placeatpc "BM_spriggan_co4" 1,1,1
Another way (which I define as slightly cheating) would be to add the journal entry itself. This is what I did, because I forgot where the spriggans appeared (I was looking west instead of east) and so I just couldn't find 'em, not even after having used the above command.
If you just can't find the things, use this:
Journal CO_10 30
It worked for me. Nothing more to say about it