Where should I look? How should I approach (ie. if they are already employed)?
If you're talking about engineering specifically, look for platforms where candidates are at least passively engaged; of which not LinkedIn is not one. I've had more success with services like Whitetruffle, AngelList and Entelo than with either contingency or retained recruitment. With Whitetruffle and AngelList, developers opt-in so you know they're at least passively interested in being contacted. Don't bother with LinkedIn unless your organization has instant brand recognition OR you're looking for Product / UX folk who tend to be more responsive on that platform.
I assume you are a non-technical co-founder. In that case I recommend reading this article: http://www.launchbit.com/blog/why-you-cant-recruit-a-technical-cofounder
It focuses on making tech people want to work with you.
My advice here is to do your homework and look for people who are somehow interested in area of your product/service. Either they work/ed in similar company, are part of a relevant meetup group or so on. LinkedIn advanced search might help here.
Next you want to reach out to your personal network and ask them to spread the word. You are looking for peer recommendations because they give you more credit and usually lead to more skilled developers. Use the power of social networks.
Besides classic job boards, take a look on this specified websites:
Hope this will help you and good luck with your search. If your developer could work remotely, I know really good people in Slovakia. Just give me a call.