The disposable cameras with flashes are a cheap source of a ready made flash unit. Some developer shops just throw them away, and if you ask nicely, they'll give them to you (so I've been told). They may be getting a little harder to find with the switch to digital though.
I actually found the flash guts from these cameras in the "surplus" section of a store here in Canada (PrincessAuto) for $0.99. A 1.5 V battery and a relay should suffice to trigger them...and they're pretty bright.

D.