In this case you could just have googled. The first result is also pretty high quality: http://flashfictiononline.com/
Of course, some would consider 500-1000 words a pretty generous length for flash fiction. Meanwhile, I'm now wondering if the form could be applied to a CYOA in any meaningful way. Kind of curious to try it now.
If you're interested in the roots of flash fiction, Hemingway is considered one of the pioneers of it. He wrote very sparsely with a 'less is more' approach that required a lot of reading between the lines. Look up In Our TIme (you can probably find any of his stuff online now) though iirc it had some pretty depressing stuff in it, like most of his stories.