You can make your own using titler. Make a white box filling the screen.
Add it to your timeline on top of your video where you want a flash;
Make it one or two frames;
Then from the beginning to end of the box change opacity from 100% (at the beginning) to 0% at the end of the white box.
You may have to adjust the length of the "white box title" depending upon how long you want the flash to last.