They can be stolen from/dropped by Undergrounders which can be found in the Underworld near Kokkol's, Tomra, etc. Once you have a few saved up, I recommend you save then use your sirens, and if after using them all you don't get what you want, reset and try again.

I think you can find sirens from a couple of other enemies in the game, but I'm not sure. I pulled the Undergrounder from the bestiary faq on gamefaqs.