Small, High dB Sound
Try "Piezoelectric sounder" or "Piezoelectric beeper" or "Piezoelectric buzzer" 

Here is a Manual from Murata who are masters of such things. A wealth of information.

These can be very small and also can be very loud. Good attention grabbing can be obtained with a warble or gliding tone. For a one time application you can get many items with these in them. Often eminently hackable. 

You can get sounders only or units with electronics built in.
 Often available as part of a personal security system so you get sounder + electronics + battery + switch. 

I could point you to specifics BUT better is to show you the general idea and let you have a look at what's available. Then you can ask locally or ask us for more input as required. 

Here is a Gargoyle image search which will give you visual leads to pages of relevance. Much better than a text search. 

Example of what's available.



Ask if more information required.



All done for you



Many such here