Ask HN: How to reach YC alumni willing to help with YC app?

The title says it. Some advice on the web suggests that it is ok to ask YC alumni for help with app. I've emailed two alumni: one promised to review our app(but so far didn't) and another one didn't respond yet. I want to emphasize that I am 100% ok with this, they are busy running their startups, and I'm basically just trying to distract them. Nevertheless, it would be great if someone familiar with YC application process reviewed it. So, HN, how do I find not busy YC alumni? Should I try to reach those who exited already?