I would try reaching out to the founders of these companies. They will recognize your email right away as a customer development effort and will sympathize with your cause since they were in your shoes one before.

One thing I've done in the past is just show up. I've found that people are more defensive on the phone since the only time we get calls from people we don't know is sales calls. Show up, ask to speak to them for 2 minutes if they are free and set up a meeting at a future time. If they aren't free, leave a handwritten note.