The Vivitar 283 is an old tried and true workhorse of a camera flash. They can be found everywhere for cheap. The downside is that they are not adjustable. If you can find it, Vivitar make a little device that turns the 283 into an adjustable flash but the little thing is around $35 - $40.
With just a 100K Linear Taper potentiometer and some soldering you can make your 283 adjustable. I've seen better 'boxes' that contain the pot than then one I made. You're the inventor, make a better one than mine.