Buying a flash unit can be expensive, but here’s a cheapskate alternative that will do the job, at least for a while. Plus, you’re recycling a disposable camera into something reusable, and saving all those bits from the landfill. Besides, once you use all of the flash out of one disposable camera, you can always build another one to replace it, and recycle the first one.
So this is a bit of a project to be honest. You’re going to need a wireless flash trigger (easily found on eBay), and a few other basic tools. Basically you’re going to strip is down, replace the mechanical release with an electronic one, and then put the whole thing back together in a way that you can plug the wireless trigger into the camera. For complete instructions on how to do this project, hop over to Instructables for the step by step solution.