I'm thinking of playing around with light activated shutter releases or flash activators, but I'm finding them scarce on the ground.

Has anyone purchased one of late, or know of somewhere I can buy one at a reasonable price?

Ideally I'd like to activate the shutter by having an object pass through a light beam - or even better a crossed beam so that the trigger is a given point in space.

You can buy kits that look like they'd do it, but I don't know much about electronics and voltages etc.