You could go DIY using parts of an old cable release, a solenoid and a timer...
Spiratone used to make an intervalometer (Spiratone Surefire) that works with a 9 volt battery. It uses a 1/16 audio jack and can be connected to just about anything.
In my head, I see a plan for something using an old cable release (for the camera side plunger and screw on thread) with a small solenoid mounted on it. All this hooked up to a battery and a MOC 3010 series opto-isolator (it's more complicated looking than it is and it isolates the potentially higher voltage mechanical side from the electronics side). On the intervalometer side, I would just put a small resistor to avoid burning the diode inside the opto-isolator...
I know it sounds complex but if you know somebody with basic electronics knowledge, they could probably whip up the main parts in an afternoon...