Try the following:

- Founder Dating (http://founderdating.com/)
- Cofounders Lab (www.cofounderslab.com)
- Entrepreneur First (http://www.joinef.com/)
- Linkedin
- Post a 'job' on Angellist to get contacted by interested people (https://angel.co/jobs)
- Check out relevant local Meetups (Meetup.com)
- Reddit
--- https://www.reddit.com/r/Entrepreneur/
--- https://www.reddit.com/r/startups
--- Or sub-reddits specifically relevant to your topic

Think of, and research, places (online or in real life) where people like who you're looking for would hang out. I know this sounds obvious, but think about not necessarily the exact interest you have in mind, but any usually associated interests that may go along with that interest as well.

If you want to brainstorm other ideas, or have other questions send me a message.

Best of luck,

Lee