Small one with central contact working on 2 AA's if your Yashi has hot shoe (this flashes usually are manual) - small and light it can sit in bag.
Downside - they usually are fixed direction, so can not be bounced off the ceiling.
Workaround: get a simple flash bracket with folding head, so you can bend it back, pointing flash up. Note - for this setup you'll need flash with PC cord.
Or maybe foldable thyristor-based, look for one which can be pointed up the ceiling. This variety draws batteries more intelligently if you use Auto mode. While they work great, they look somehow weird on RF's. They match SLR's, but if you don't care how it look - why not?
I like flashes with PC cord so they can be used on flash brackets. Moving flash away from lens minimizes chance of red-eye, in case you don't want them in pictures. Foldable flash and/or bracket with foldable head allow bouncing.