Here's a list of enemies that can drop the Alert item. The chances vary somewhat, so I've catagorised them from most likely to least likely. Don't ask me where any of them are, because I haven't the foggiest.

*Eyes
*Last Arm
*Searcher
Gr. Dragon
Y. Dragon
Arachne
Araneid
Undrgrnder

* Drops nothing but Alerts.