I think the question should be the other way around: How does people interested in Open Source development find you and your projects? My answer would be: be visible and provide easy ways to get engaged.
Being visible means to have a good homepage which explains the project as good as possible, write about it on blogs, news pages, social networks, go to conferences, etc. Release early & release often. Also it is important to work in the open. Even if you are alone. Have a public repository right from the beginning, make pull requests so that people can see the code, the work and the workflow. This makes it more likely that people will join you.
Providing easy ways to engage means to have public mailing lists, maybe a forum, issue tracker and an IRC channel. I would also recommend to focus on open and free communication channels.
Then, if people find you, you need to start to build a community. Response to emails, forum entries, issues, etc. Tell people how they can help. Review pull requests in time and guide the people to other issues by asking them to review your or other pull requests. Trust people by giving them merge rights as soon as possible.
Hope this contain some useful tips.