It would be useful to know the role you'd like your cofounder to play (i.e. technical, business, etc.), but here are some general ideas. Finding a good cofounder isn't easy, and your relationship has to be, or become, very close to succeed. To find one you'll need an idea that people can be passionate about.
That being said, here are some places to look:
1) Ask your friends (obvious, but a good place to start)
Weave app (https://itunes.apple.com/us/app/weave-meetup-local-business/id788443696?mt=8)
Sub-Reddits specifically relevant to your topic
3) 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.
More specifically, if you're looking for a technical cofounder:
1) If you're specifically looking for engineers, look up local 'hacker' / maker spaces that may exist in your area (see here: https://wiki.hackerspaces.org/List_of_Hacker_Spaces).
2) Look up local universities and email the chairperson of whichever programs relate to the type of cofounder you need (i.e. electrical engineering, marketing, etc.). You could even possibly go there in person and put up a sign on a bulletin board.
If you'd like a more specifics let me know,
There can be 5 ways to find a co-founder. Trust me, you will definitely find one. Below are those:
1. Post it on your LinkedIn and let the connections that you are looking for a co-founder.
2. Post it on your Facebook wall and share it in different groups.
3. Post it on your twitter and also request your followers to retweet it.
4.Call all your friends and family members and let them know that you are looking for a co-founder.